Long story short: during the playtest for the book it came in (Ultimate Combat, IIRC), there were several posters on the PF forums who did some heavy number crunching and fairly soundly demonstrated that the Gunslinger became more dangerous and unreliable as it gained levels. This hinged mostly on the misfire rules combined with iterative attacks; when you make 3 attacks a round, every round, the odds of eventually getting a misfire rapidly converge to 1. There were other issues, but this was the key one. Some of these posters were...very uncivil about their criticism, but others were perfectly reasonable and even offered suggestions for how things could be changed.
Almost everyone who criticized the class was banned from the forums, and the class released with no changes whatsoever. Naturally, non-fans were not amused, but even some loyal customers took this somewhat poorly. It was a minor point in the grand scheme, but it rather pointedly revealed Paizo's attitude toward playtesting and especially numerical analysis.