I sent in my 6 year old A2 because someone dropped it (not me!), chipping the lens and leaving a flat spot on the bezel. A surefire rep replied to my email within an hour with an RMA. A little over a week after mailing it from Florida to California I got it back with a new bezel and lens installed, free of charge. Not only that, but I sent it without batteries and they sent it back with 2 new batteries in it. That's great service!
Other times I can think of:
P91 lamp - intermittantly inop
P91 lamp - bulb loose and the replacement had the bulb thoroughly potted in the reflector, which I've never seen before.
X80 lamp - lower output, too yellow
A2 tailcap - broken tabs
Z57 tailcap - stuck on
V82 & V21 holsters - stitching coming apart
In all these cases they sent a replacement with no questions asked. They didn't ask for proof of purchase or for me to send back the faulty part.