Not entirely sure if this was intended, but Lich Bane procs upon beginning to cast a spell, not after the full channel. I don't just mean on spells that are meant to be channeled like Fiddlesticks's Drain or Malzahar's ult. It will do this on any spell, even with a cast time of 0.1 seconds.
1. Playing Veigar in a custom game, bought Lich Bane.
2. Was trying to last hit with Q, sometimes minion would die before Q finished channeling.
3. After experimenting, turns out the auto-attack that was flying through the air was hitting for Lich Bane damage, cancelling the spell because the minion would die.
I know it seems like a small issue, but I got fairly good at proc'ing LB and not spending cooldowns. I was dealing 600~ damage to minions without casting anything. I would just wait until my auto-attack was almost at the minion and press Q.