[Suicide Junkie]

My guess was it had something to do with Seeker A targetting seeker B.

1) Seeker A hits seeker B.
2) Seeker A is removed from the battle.
3) Seeker B is killed by the damage from A, and is removed from the battle.
4) All seekers targetted at B are removed from the battle, including A.
5) Since A is already gone, it can't be deleted a second time and the replay generation chokes.

Just my theory, based on only a little evidence.