Minor spontaneous disassembly

I'm wondering if anyone else has had a trigger pin shear off or walk out while firing. I'm not sure which happened because I couldn't find the pin afterward. I was at the range zeroing in my WASR and after about 20 rounds the trigger felt wonky. After making the rifle safe I looked it over and immediately saw the trigger assembly wasn't lined up and the trigger pin was missing. I've never removed the FCG (until I brought it home from the range today). It's a tapco trigger. Is this a common failure? Is there anything I can do to prevent it from recurring?