If you kill your opponent hero with Energy extraction the opponent hero's hp goes down to 0 and then the game freezes.
All time I saw this bug was using Energy extrction on Derkan (and derkan was only ally on my side).

You can see this in the following games:
A1 painfulsmile vs Dexter99
A1 painfulsmile vs BlackSorcerer (first game)

Both time I did not wait too long. Only 1 or 2 minutes and then left the game and got a lose for that.

Hope that helps.