Mordekaiser's Ult
I feel like everything about this thing is broken. Ghosts don't move to where you tell them to or randomly start attacking targets when you're not in combat, Dragon ghost suffers from all the bugs of regular ghosts as well as the weakness that it doesn't fast travel with you. That itself wouldn't be as big of an issue except for the fact that if you're playing Mordekaiser top lane, your dragon ghost will be dead before it ever has the chance to have any impact top lane.
At this point I consider it a broken ability that needs a fix asap. Compare it to Yorick's minions and you'll see the difference in quality.