Our group tries to correct the mistakes with initiating passives (at the wrong time or forgetting to) if spotted in time. If not, we roll with the punches.

How do you guys handle play restrictions that were not met?
Events or attachments played when their conditions just didn't meet?
Discard or back to hand?

If spotted when just played (or put into play), back to hand (since the card cannot be played, it should have stayed in hand). For an attachment spotted later, discard pile.

For us, we try to correct a mistake as long as it didn't influence a decision to be made(basically we all agree on it). Attachments must be returned to hand immediately if it is illegally attached. If it's been illegally attached for some time, it is discarded. Events are returned to hand and not played yet if the target chosen is illegal. We don't make anyone choose other targets. If the even does not choose a target, then it needs to stay played. That's just us though.