It's not a well-known issue, but there have been a few reports of macbook pros (2010 - 2015) having loose or wobbly keycaps. Now this isn't something you would really notice unless it bothered you (especially since you would acclimate to it over time), but since I couldn't find any information about this online I'm posting my findings. Perhaps a more significant issue is that when you have loose/wobbly keycaps there's also a high chance your keys will exhibit a "click/pop/crunch" noise when striking the key at certain angles along the edge (which will be explained shortly). This is also something that tends to affect the retina models more than the non-retina unibody models, as the 2010-era unibodies have larger key travel and deeper catch grooves on the key, making this less of an issue.
This likely does not apply to the M1 models since they redesigned the scissor mechanism to be more stable, hence why I'm posting this in the intel mac forum.
First note that different models of macbook pro each have their key type, which you can readily find online [1]. As an example, 2013-2015 MBPs have keyboards from two different manufacturers, with an "AC06" or "AC07" keytype. It's not clear if one keytype is more susceptible to this issue than the other (data from people would need to be gathered) but it's quite possible (as a reference point, mine was AC07 and exhibited this issue on almost every key).
The root cause of the issue seems to be the positioning of the metal support pins on the keyboard. This can be confirmed by swapping both the keycap and scissor mechanism from a known "good" key into a known "bad" one. You will find that the wobbliness (and possibly crunching) persist, indicating that neither the keycap nor the scissor mechanism themselves are the origin of the issue. My guess here is that either as a result of manufacturing tolerances or simply use over time, the metal support pins become slightly misaligned. As a result, they no longer hold the scissor mechanism tightly and the misaligning permitted results in keycaps that can wobble. It also explains the crunchy/clicking noise when striking the key at weird angles (e.g. when depressing the key with a slight rotation) since I assume there's a phase transition from the misaligned state permitted when the scissor is raised to the state when the scissor is lowered (where no misalignment is possible simply due to the geometry of the thing).
How can we fix this? The fix seems to be rather simple: carefully remove the keycap and scissor mechanism, and gently nudge the pins. You do not have to nudge them very much, simply using your fingernail and applying moderate pressure will do. The astute reader will be asking a few questions:
* What is the best way to remove keycap/scissor? You can likely find videos for your particular model, but the general principle is this: first remove the keycap by pulling in a diagonal fashion. It's important that you do not pull straight along the edge, since you don't want to rip the scissor mechanism out. As a concrete example, for an AC07 key mechanism I pulled from the top left corner. This way you can release the clip without entirely yanking the scissor mechanism. Try to peel the key off rather than simply yanking everything upwards: your goal should be to release the key while keeping the scissor mechanism still attached to the keyboard. Once done, the scissor itself can be easily removed by unhooking the pins. Again this depends on your exact model, but from AC07 you can pull downward to get the scissor free from the bottom two metal holders, then once that's done the rest lifts off easily.
* Which way do you nudge the pins? I'm honestly not 100% sure, I nudged both top and bottom pins outward (on the reasoning that it would hold the scissor tighter) and it seemed to be an improvement on all the keys but some improved more than others. It's likely that any sort of misalignment results in the wobbling/crunchyness, so you may have to play around a bit with nudging along horizontal or vertical directions. If you are very sensitive to these things you may be tempted to keep fiddling with it. I'd probably say to cut your losses and stop at a point where you have improvement over the previous case, since each time you fiddle with it you are bending the metal slightly out of shape, and you don't want to get to a point where the the scissor no longer fits.
* Risk assessment of this: I believe that this fix is negligible risk. The scissor mechanism is actually quite hardy, I had an old 2009 mbp which I went to town on ripping out keys, and even after rough treatment of yanking the scissor itself remains functional. Even if the scissor comes apart, you can put it back together carefully (mind the orientation, that's the most tricky part of reassembling). Worst case if you destroy the keycap or scissor entirely, new ones can be had for less than a dollar. The only part you want to be careful is that you don't bend the pins too far, as otherwise the scissor will not lock into place. Also be careful not to damage the rubber nub.
[1] https://www.realrecyclers.com/en/info/keycaps-exchange.html
This likely does not apply to the M1 models since they redesigned the scissor mechanism to be more stable, hence why I'm posting this in the intel mac forum.
First note that different models of macbook pro each have their key type, which you can readily find online [1]. As an example, 2013-2015 MBPs have keyboards from two different manufacturers, with an "AC06" or "AC07" keytype. It's not clear if one keytype is more susceptible to this issue than the other (data from people would need to be gathered) but it's quite possible (as a reference point, mine was AC07 and exhibited this issue on almost every key).
The root cause of the issue seems to be the positioning of the metal support pins on the keyboard. This can be confirmed by swapping both the keycap and scissor mechanism from a known "good" key into a known "bad" one. You will find that the wobbliness (and possibly crunching) persist, indicating that neither the keycap nor the scissor mechanism themselves are the origin of the issue. My guess here is that either as a result of manufacturing tolerances or simply use over time, the metal support pins become slightly misaligned. As a result, they no longer hold the scissor mechanism tightly and the misaligning permitted results in keycaps that can wobble. It also explains the crunchy/clicking noise when striking the key at weird angles (e.g. when depressing the key with a slight rotation) since I assume there's a phase transition from the misaligned state permitted when the scissor is raised to the state when the scissor is lowered (where no misalignment is possible simply due to the geometry of the thing).
How can we fix this? The fix seems to be rather simple: carefully remove the keycap and scissor mechanism, and gently nudge the pins. You do not have to nudge them very much, simply using your fingernail and applying moderate pressure will do. The astute reader will be asking a few questions:
* What is the best way to remove keycap/scissor? You can likely find videos for your particular model, but the general principle is this: first remove the keycap by pulling in a diagonal fashion. It's important that you do not pull straight along the edge, since you don't want to rip the scissor mechanism out. As a concrete example, for an AC07 key mechanism I pulled from the top left corner. This way you can release the clip without entirely yanking the scissor mechanism. Try to peel the key off rather than simply yanking everything upwards: your goal should be to release the key while keeping the scissor mechanism still attached to the keyboard. Once done, the scissor itself can be easily removed by unhooking the pins. Again this depends on your exact model, but from AC07 you can pull downward to get the scissor free from the bottom two metal holders, then once that's done the rest lifts off easily.
* Which way do you nudge the pins? I'm honestly not 100% sure, I nudged both top and bottom pins outward (on the reasoning that it would hold the scissor tighter) and it seemed to be an improvement on all the keys but some improved more than others. It's likely that any sort of misalignment results in the wobbling/crunchyness, so you may have to play around a bit with nudging along horizontal or vertical directions. If you are very sensitive to these things you may be tempted to keep fiddling with it. I'd probably say to cut your losses and stop at a point where you have improvement over the previous case, since each time you fiddle with it you are bending the metal slightly out of shape, and you don't want to get to a point where the the scissor no longer fits.
* Risk assessment of this: I believe that this fix is negligible risk. The scissor mechanism is actually quite hardy, I had an old 2009 mbp which I went to town on ripping out keys, and even after rough treatment of yanking the scissor itself remains functional. Even if the scissor comes apart, you can put it back together carefully (mind the orientation, that's the most tricky part of reassembling). Worst case if you destroy the keycap or scissor entirely, new ones can be had for less than a dollar. The only part you want to be careful is that you don't bend the pins too far, as otherwise the scissor will not lock into place. Also be careful not to damage the rubber nub.
[1] https://www.realrecyclers.com/en/info/keycaps-exchange.html