Harold Unites the Elves / Mazkwood Nexus interaction
My opponent has Maskwood Nexus on the Battlefield, then played a Herold Unites the Elves. He would use the Unites the Elves trigger to pull another Herold Unites the elves from his graveyard. This should not have been possible since the Maskwood Nexus text says that it only affects creature cards, but Unites the Elves is an enchantment card.
He ended up making it an infinite loop, not sure how he was getting the Harold Unites the elves back into the graveyard. He went through most of his deck before pulling the card he wanted.