I've only seen this one once.
While playing some AIs as the Torchbearer, I tried to fireball a nearly-dead Rook who was teleporting back to his base. The fireball went off just about when he teleported, but instead of streaking to his health crystal and killing him as I expected, it just sat there in mid-air for 15-20 seconds, then disappeared. I've never seen this happen before, so it must be an unusual combination of circumstances that caused it.