If it happened once every 10 years, yes I would.
Do you think all ATMs should be banned on the off chance that occasionally one of them doesn't work correctly?
Fair enough, of course if the £20 is the start of a run of mistakes then at the least of it you'd be £20 down before you started taking action.
As to banning ATMs a bit of an extreme reaction. The banks are quietly removing them anyway leaving only the ones which charge for their use. I expect you're happy with that.
Given the way that cash is, quite literally, going the question will soon be redundant and we'll be relying on the integrity of the banks not to rip off the consumer more than they do now.
That aside I'm sure that facial recognition will be fine and that if it ever gets to 99% accuracy the 1%ers who get minorly inconvenienced will also write it off to experience.