Now there is a question. If you softball an encounter by say, changing tactics and allowing the pc's to get the upper hand, is that fudging?
I should hope by now that my answer of "not even slightly" is well on record. Unless--and again this exception is one I've said several times--the situation has already been very clearly laid out to create specific, and factually-supported, player expectations. The easiest example being "the characters acquired a guard duty roster"--that should be
pretty specific about the disposition of guard forces at various hours.
Party: We went through this whole fight and it was all for nothing! Elminster just came in and took care of it.
Change "Elminster" for "the DM" and you have
exactly how I feel about fudging. Add to it intentional deception and I consider it a recipe for disaster. I don't believe
anyone is smooth enough, even if they only do it rarely, for me to never
ever notice. So as far as I see it, it's not a matter of "if I find out," but "when."
Even when I never speak up about it, I almost always pay fairly close attention to how the DM runs the rules. It's not really a matter of being critical, and more that I just tend to learn the rules myself, and if there's a question or confusion, try to help. Can't really help if I'm not paying attention. Even if I never see the numbers themselves, and even if the DM always uses purely custom-made content (which I also consider pretty unlikely), I'm not really convinced that the "illusion" can be maintained indefinitely.