Eventually in a system like that they may refine their procedures and then you get dinged essentially...
"The guy who went to jail" could be unvaccinated (or even infected) and presenting other people's certificates to enter an area for vaccinated people only (e.g. hospitals) where he might have endangered other people's lives; that's something that might be deserving jail time. I was vaccinated however, and by all means had the right to enter that shopping mall; I just wasn't able to prove it to the imperfect system that was there to check.
Digital IDs can't be faked. The only way to fake them would be to convert them to physical (what you did) and hope that the physical ID gets accepted.
(Digital IDs indeed can’t be faked but usually they are a part of a process that can be easily bypassed by using something that presents itself as a valid Digital ID even if it’s not.)
Credit cards are a great example: they can't be faked, however while the cryptographers are sitting on their high hill and patting themselves on the back for doing great job, the credit card fraud rings billions of dollars every month. It doesn't happen because of fake cards -- it happens by exploiting the flaws in the whole process that a (non-fakeable) card is a part of.