The whole thing is riddled with people making poor decisions, ratruns, shortcuts etc etc etc to be watertight, you never could make it foolproof.
I see the vast majority at work, hundreds of people every day, choosing not to wear a mask. Those people are in close contact for hours on end and yet they carry on regardless.
In the previous lockdown, my wife saw former workplace staff coming back from the India / Pakistan via Paris to circumvent restrictions.
There is no perfect scenario....some people are forgetful, some are blatantly putting two fingers up to it all, some simply don't see themselves as responsible...it's someone elses problem.
Some...alarmingly many, people are the problem.