For the record, I have been working a bit on my WIP SCPs, but I just had another idea I thought might have some potential:
An arcade game cabinet for some old top-down shooter (bullet-hell style, maybe?). The cabinet itself is harmless and the game works normally, though people that play it are wholly convinced that there are rules to it that aren't really there, e.g. missing shots or using bombs makes you lose points, or that your own bombs can kill you. None of these rules are ever mentioned or actually work as the player perceives them, but it may take a bit of convincing for players to believe otherwise.
After the player quits playing, however, is where the game's REAL anomalous effects take place. People that have played the SCP will still suffer from the same "false rules" syndrome; being fully convinced of the existence of extra rules or restrictions present in various known rule systems. At first the effect is minor, manifesting mainly in other games, sports, or recreational activities, but over time it begins affecting their perception of things like traffic laws, dress code, etiquette, or workplace rules (including the likes of containment breach response procedures). I realize this sounds kinda like that shaving kit that made you indecisive, but these rules aren't binding; they may still act against these "new" rules (though they will believe things have always worked like that), though they will still react to such actions as they would to breaking a regular rule or other such action.
Example, that pizza box SCP is free to use by staff, but someone under this SCP's influence may think staff are only allowed two pieces a day or something. They might still sneak more pieces of pizza and stuff, but they would try not to get caught and show guilt when actually caught, even though they haven't actually done anything wrong. The effects aren't permanent and infected individuals can be "cured" by proving these new rules don't exist, but you'll still have to deal with their delusions until then.