If the play was not valid, the challenger wins the round. Players may not challenge themselves. When a vowel is played, the order of play reverses. When someone plays a vowel, that player also adds the same vowel to their collection unless they already have it. If there's only one tile of a certain vowel in the pot, you may play that tile, but you don't get one for your collection.
Collections persist across rounds.
When a round is over, players keep their vowels. Even when collections persist across rounds, when anyone wins by collecting five vowels, everyone turns in their vowels. When someone plays a vowel they already have in their collection, they lose the one in their collection. Players may start a new word that intersects an existing string, but only if all existing strings are complete words of at least five letters. A player wins if they create a closed shape with a hole.