Master Yi

If a DoT ability (i.e. Ignite, Torment, Malefic Visions) has already been applied, you will still take damage while untargettable.

good

all Dot damage and DaT like Zed's deathmark and Vlad hemoplague do damage even if you are untergetable. Apart from that, there are man bugs with Alpha strike. Many targeted abilities hit you while you should dodge them...

Unless you are describing some rare occurrence where you instantly did a thousand damage to yourself with Q... not a bug

I think we need more info. Is someone attacking you while you're using Q? Are you by yourself and just hitting minions and when you Q you just all of a sudden die? Did Karth ult? Did you have a damage over time affect on you? Elaborate please.