You're not allowed to build the same building more than once. I noticed that this rule is easily overlooked, especially in 7-player games and when the table is not that big (and so the cards need to be partly stacked).
If a player has built a building twice, but it is only spotted in the following round(s), what can you do?
- Discard the card
- Discard the card and give the builder 3 gold
- Let the builder choose: discard card for 3 gold OR use the card to build the wonder
- Keep the building
- … something else?
Of course there might be other side effects, as when the builder (or neighbours) already made use of the illegal building. It could be very hard to retrace everything, so I guess it'd be best not to try to undo any side effects.