I bought an AK bit over a year ago and didn't realize that the previous owner ground down parts of the trigger which ended up making the trigger not reset. I had to completely replace the entire trigger assembly and that's when i found out that the parts were ground down on the bottom edges. (You couldn't see from looking into the rifle from the top.) Thank god that he didn't cause it to go full auto because the first time i took it out shooting i was out shooting with an LEO friend. I do feel a little proud that i was able to replace the trigger myself and now have a perfectly functioning rifle.
Yep, I always assume someone may have messed with the trigger. That is part of the reason I replace them, and with a much better than stock. I hate firearms with triggers I don't know. I always look for stress cracks or signs of pressure issues also.