Normally, I'm a very forgetful person, but letting debt pile up is something I never allow myself to forget.
When I lived in Germany everyone paid their parking tickets religiously. In the UK you'd also have your car towed away until you paid your tickets with interest plus the storage and towing fee.
j/k Their license will be suspended and the fee will be treble after the expiration date. And they still won't know until they get pulled over or run a license check.
edit/ I've been there personally and I was simply unable to afford them, it had nothing to do with forgetting. Thankfully I never had my car towed as a result.