AD. exploding siege tank, it was actually a fix in one of the patches. Some players discovered that you could land a building on a tank if you time the landing right with switching to siedge mode, making the tank unreachable for meelee units (which were the kind of units usually used against the tank). It created an unfair advantage, so Blizzard removed it by making the tank explode when landed on.

Funny fact: if you landed a building on your opponent's tank, you could actually get a label "Kills: 1" on the building :D.