It's very likely a user error. I've used these Google forms for a long time, and I've never seen a submission not go through. An error in the way data is handled in the spreadsheet is of course plausible and could lead to submissions disappearing, but if the raw data that comes directly from the form does not have the submission of a specific user, that user probably didn't send it. Why that is, I have no idea. It could be anything from forgetting to click the last button, to having a network error or something.
And the fact that there have been multiple cases of these, is probably due to the fact that every time you ask tons of people to do something, small percentage will do something else. If you ask 100 people to press "Button 1", about 2-3 of them will press "Button 2". That's a statistical fact.