Just a couple of anecdotes - the first being cash machines where I input the PIN using the traditional buttons on the numeric pad, then i start using the touchscreen to make other choices. Pick one or the other, not a mixture of the two. Or self service petrol pumps that end up covered in grease and dirt, because they're in that kind of environment.
Once i was waiting in line at the Eurotunnel and the person in front of me was clearly struggling. It took them about five minutes to get through the registration process and they kept leaning over out of their car window, stabbing away at the touchscreen with their finger appearing somewhat frustrated. I assumed they must have had trouble finding their booking.
When I got to the booth myself I pressed the touchscreen and it was immediately clear that the calibration was off, by about a couple of finger widths - this was enough to cause the problems for the user before me, as they were trying to type in their booking reference but it would always choose the wrong digits. So when I typed in my booking reference I moved my finger a little to the side to account for the calibration error. Easy! Well, not so much if you're not technically inclined enough to recognise this kind of issue when you see it.