The creature that has me asking this is the Goblin Acolyte of Maglubyet (MM2). Its aura stops hit point regain. Now this kind of activity (hit point regain) usually occurs when "forced" by the party healer.
So if no one knows about the aura's effects until they are enacted, the healer says "spend a surge" you do and you do not regain hit points.
This isn't the aura where you notice you don't hit as often. Or take random damage because the monster is thrashing around. These I wouldn't mind finding out when I'm inside it.
I'd however be pretty pissed to find out that I wasted abilities which are limited to 1x or 2x per encounter or even day (Second Wind, Healing Words, etc.) on an aura that completely negates their effect. Since the standard rule is "I know what it does" and I wouldn't have used it would I know, I'd suggest to at least don't expend the use (the action is wasted though).