The "ignore sneak speed penalty" part of this set stops working sometimes. I've noticed it right after going through portals that NPCs send you through while questing. It just stops working, just like how dark stalker used to stop working when you died.

You can fix it by just taking off a piece of the gear set and putting it back on, so it's not nearly as annoying as the dark stalker bug, but still, thought you should know. Sending the same in a bug report.