Keep in mind that the teath of the gears are always meshed with the other gear. What moves in and out of position or the dogs on the sides of the sliding gears.
A bent fork would keep the gear from sliding all the way over so that the dogs fully engage. In this case, you'd feel the resistance (and some chattering) in the shifter as the gears would push back through the forks. In this case, I would expect that the bike wouldn't stay in gear (or never get into gear) and the sound would go away once you took pressure off the shifter. Either that or the dogs are so worn down that they just barely reach the locked gear.
Generally, the teeth don't wear down enough to allow the gear to just slip will it's in place. Since the teeth are always engaged, they don't take a big hit when you shift. It's the dogs that get beat up.
Not that it matters, since either way you need a new gear. Also, if the fork is bent, you probably need a new gear due to the damage that was caused by partially engagin the gear. Worn dogs is generally why bikes pop out of gear as well.