Please note a similar thread does exist, but it was not set out the proper way, and although the comment below does exist on that thread, I thought I would re-post the thread with the correct formatting. In the other post the person had a problem with using Sion's E directly after his Q, apparently making it so it didn't kill the minions when in should have.
I encoutered this bug on live while playing Cho'gath in a game with Sion jungle.
The sion e'd a melee minion just as I feasted it, which should have done 1000 true damage, but only took the minion down to 1/4 health. This was on the live servers by the way.
**HOW TO REPRODUCE**
1. Use Sion's E "Roar of the Slayer" on a minion
2. Simultaneously use Cho'Gath's Ult
3. The minion will survive the 1000 true damage from Cho'Gath's ult, with about 1/4 of its health remaining.
Conclusion: Sion's E does wierd things to Minions.