I was messing around with stacked entities, when I tried to create a spawner that spawned one long tower of creepers in the middle of an arena. The spawner worked, but even before I went near the spawner, this tower of creepers was already there. You can't place blocks where it is, it can be pushed by pistons, it doesn't react to you or move AT ALL, the top creeper kept flickering, AND MC edit doesn't even detect it as an entity. Re-logging doesn't work, and the only way to get rid of it is to destroy the spawner, THEN re-log. I have NO idea why this happens, but it's not right.
P.S. If the location isn't set, this doesn't happen, which is really strange