I guess we're to assume that the rule was bent for Bob due to his humiliating anatomy. No doubt the producers of the film also were desperate to avoid showing him topless too.
Since Ed Norton's character shows a great deal of sympathy towards Bob throughout the film, (e.g convincing him to stay on the porch after Brad Pitt's 'personality' told him he was too fat) it would make sense that he would allow Bob the dignity of keeping his shirt on. It's also possible that Bob's breasts were classified as some kind of weapon! Best keep those babies under wraps!
Bob actually broke two rules of fight club. The first one is about the no shirt, no shoes thing, and the other was when bob sees Ed Norton's character on the street for the first time after he stopped going to "group therapy" with a bucket of chicken. remember that part? anyway, bob starts telling Ed's character about fight club, even the rules. That was the first two rules, DO NOT TALK ABOUT FIGHT CLUB!
First of all: no-one wants to see Meatloaf with b**ch-t*ts. Second of all: Even if Bob did talk about fight club, it's not a slip up of the movie, it's intentionally a slip up of Bob. Please read the rules for submitting slip ups and follow them.
Like everyone else has said it was the man boobs thing, and maybe bob did break two rules, but as Brad Pitt points out during the film, the club is getting so big that a lot of members MUST have been breaking the first rule of fight club, not just bob SO LEAVE POOR MEAT-LOAF ALONE!!!
Pitt&me: Bob didn't really break the rules, he very circumspectly mentions a certain group whose first rule is not to discuss it - kinda sounding out "Tyler" to see whether he already knows. But whatever, Tyler later complains that lots of people have been breaking this rule (commenting on the number of new faces in the crowd), so it's not really a slip-up.