Maybe admonishment by Admin and revoking member's permissions after a few violations. Maybe add a few more questions to the uploading process. They would act as a reminder trigger. "Did you verify card numbers?" "Checklist source?"
Then there's also source problems. I've used companies (Topps, Upper Deck, Panini) websites and they aren't always correct. I believe they produce checklists during the development stage. The finalized numbering sometimes changes and they don't bother to change the original checklist.
Then there's Beckett's and hockeydb.com. If you bother to read the set description, Beckett's let's you know when they've added numbering to NNO cards. But, hockeydb.com does not. They number everything regardless of NNOs.
Someday my cards may double in value and then be worth half of what I paid for them.