It's pretty rare, really, on the 750, more common on the 500.
There's 2 things that can cause it:
1. Loosened shift drum pawl retainer plate. This is a wordy description of the screw and retainer plate that holds the pins on the shifter drum: if abused, this screw can work loose, letting the pins fall out of alignment. When this happens, the shifter can't pull the drum all the way into the gear position, so it mimics a bent shifter fork. This can be accessed under the cover on the left side where the shifter enters the cases. You'll see it immediately if you remove this cover: it's toward the top, and the screw is right in th emiddle. It was dimpled in the locked-down spot when built, so if it's not tight, it will be obvious. The good news: this is more common than bent shfter forks, easier to fix, and does no damage to the internals of the tranny, other than poor gear engagement.
2. Bent shift fork. This is far more common in 2nd gear than 1st, though, due to "racers" shifting without the clutch, or from shift drum wear on high-mileage bikes (like mine!). New parts will fix it right up. The bad news: you gotta pull the engine to do it.
