Dunno what they do with iPhones these days, but back in the day with Macs, they'd have two different streamlined Mac Pros hooked up to the same presentation projector, and they could switch to the backup in an instant if there was a problem with the first.
The machines had no crud on them, but apparently they were actual production spec machines, albeit top of the line. Undoubtedly the software was tailored to make the talks look good, and it was rehearsed many times before the presentation. And in the case of a fail, then they'd flip a switch and go to the second one in an instant. I've seen Jobs do that on stage, and he even acknowledged the importance of a backup machine when the first one hung, to laughter from the audience.
My favourite screwup was with a Canon camera. He couldn't get it to sync so he got pissed off and literally threw it into the crowd. I thought he was going to hit somebody in the head with it, but instead somebody got a free camera.
BTW, off topic, but one giant clue we all missed was a presentation where Jobs typed in a tracking number for a package and showed how he could get the tracking data right in the app he was using at the time. I don't even remember the app, maybe in Dashboard? I dunno.
However, the importance of this little tidbit was that it was actually a package from Intel containing x86 CPUs going to Apple. Nobody picked up on this, and shortly afterwards, Apple announced the switch to Intel.