Linux jumped from 2.6.39 to 3.0 because the numbers were getting too big. He also decided that as the middle digit wasn't being incremented, he'd drop it.
I really wish he would've gone with date-based version numbers. 3.X is eventually going to get "too big" and it will be the same (non-)problem.
I remember seeing 2.6.42 on my ubuntu 10.04. May be it was kind of an alias for some 3.x version.
Yeah, some software assumed that there must be three digits in version numbers so 3.X kernels were sometimes called 2.6.X+40 which means 2.6.42 actually is 3.2.