1) Sun Quan, Lord of Wu [Portal Three Kingdoms]. Its ability gives all creatures you control horsemanship, yet a creature that didn't have it (in this case a Marauding Knight) was able to block one of my creatures (a Vedalken Entrancer).
2) Chainflinger [Odyssey]. It was able to use its threshold ability (it was being used by my opponent) with an empty graveyard.
3) Pledge of Loyalty [Invasion]. It's a creature enchantment yet when played it becomes a standard global enchantment, making it pretty useless.
