This afternoon my wife tried to play a green 8 on top of a red 2 in Uno.
Obviously a mistake on her part and I could find nothing in the rules to suggest that there was a penalty so we allowed her to withdraw the green 8 and play another card.
The rules do state that a Wild Draw 4 play can be challenged, but no other misplays are called out that I saw.
Is anyone aware of this situation being covered, perhaps in a previous ruleset, or maybe I'm just overlooking it (It's been known to happen!)