Thomas Shey
Legend
Oh, boy! I was thinking of stealth rolls before I finished reading your first sentence. I try to take a page from video games like the Metal Gear series. Your first flub on a stealth roll might pique the interest of a guard but by itself it isn't going to bring the whole mission to a grinding halt. I also avoid having them roll too many times. i.e. They can get around the area with one stealth roll rather than having to make a roll every time they move.
There's a lot of fixes you can do for this sort of thing, but relatively few games give you any guidance to doing so, let alone have mechanical support for avoiding the problem.