And although there is a circumstance were Steam could lock you out of your games, I imagine this is not an issue for average players just playing their games. My understanding is that some kind of cheating or hacking would have to be involved. Is there another example?
Unfortunately this is incorrect. Steam can lock you out of your games for all sorts of reasons. Below is just an example of one other but there are many more.
I've used Steam from the beginning of its Half Life 2 launch days and not had problems with the service until this year and have since decided not to purchase or use anything from them again.
The cause of this was Steam's lack of proper labeling for Ubisoft games that require Uplay. Typically any and all requirements on a game are (and should be) listed in the System Requirements at the bottom of the game page on Steam. With games by Ubisoft requiring Uplay (a second DRM program that is required to be both installed and launched in order to run the game) Steam has decided to hide this feature off to the right hand side of the game page, under controller support features and the ESRB rating (a frivolous area unnecessary for most viewers to pay much attention to).
It's important to point out the invasive nature of Uplay and it's bad reputation as well:
http://www.rockpapershotgun.com/2012/07/30/ubisoft-respond-to-uplay-security-drama/
http://www.forbes.com/sites/ibm/201...he-cloud-is-boosting-global-travel-and-trade/
The Uplay requirement has until this month been listed in light grey font and is still listed in extremely small type atypical when compared to other 3rd party requirements such as those listed by MMO's or specific games that have features that will effect the typical user experience of a game such as Early Access games or hardware specific requirements (see Dark Souls, or The Witcher 2). In those instances the requirements are highlighted front and center at the top of the page with a bright orange or green box around them. Unlike GoG which has a 30day return policy, with Steam all sales are final from the moment of purchase.
Contacting Customer Support which until this year was the first time I've ever had use them is a wretched experience. All communication is through email which takes a minimum of a week and is typically just answered by a bot with preprogramed answers responding to the key words of your email. The return mail would be laughable for it's wildly off topic responses if the process weren't so infuriating.
So to have an actual human response will likely take several weeks as it did with me and then you will be equally dismissed. "All sales are final and no attention is needed to clarify any information on the game page etc." In other words, 'thanks for your money and go **** yourself.' If you are unsatisfied with this and contact your credit card company to freeze payment Valve will lock you out of your account until the money is paid.
This kind of blatant disregard and disrespect underlined the importance of supporting alternatives to Steam and it's near monopolistic control over digital downloads. I've started buying from GOG and the app store as well as supporting some DRM Free Kickstarters for iOS and OSX. My hope is apple unveils an apple tv with the graphics power of something near the wii or better and opens the platform to games but who knows. For now dealing with Steam Support has made me ardent supporter of just about anything else. I might even pick up a console finally. (Haven't done that since the SNES days)