So I've run across this problem on several occasions now. My friends and I will be playing on my server. I've done no modding of files or the database. We'll find a vehicle and it'll be damaged. We'll source some parts from industrial buildings and go to repair the vehicle. Usually how it works is the first or second thing we try to fix works fine, but then anything after that doesn't actually get repaired, even though the part disappears from our inventory, and we see the message saying it was successfully repaired. Same goes with fueling up helicopters. Cars and trucks seem fine, but helicopters are hit and miss with refueling. We could dump 10 jerry cans into an empty helicopter we've found and it doesn't even register on the fuel gauge. It's really weird. One other glitch that happens, and it's very rare. I'll find a vehicle, get in it to see what's damaged and how much fuel it has. Then when I get out suddenly I see the windows and body sort of "pop" and everything looks perfect. Jump back in and everything is suddenly at full health and full fuel.