Well, I am no chemist, but I do know that it was used as the initiator in WW2 German bombs. I also know that the main reason German WW2 electrical bomb fuzes are generally not removed from the fuse pocket by RAF bomb disposal even today is that the picric acid seems to form crystals on the locking ring thread, which can lead to vaporized EOD personnel when turning the locking ring causes friction on the crystals. If you see the bomb fuze design, the actual picric acid pellet is at least about 3 inches down from the locking ring, so it would seem the crystals can form some distance away. The procedure generally is to trepan the whole fuse pocket out of the bomb case and blow it up. I would think it would be, er, irritating, to turn a gas cap and have the gas tank explode. I don't have the faintest idea if it would form crystals when mixed with fuel and I wouldn't volunteer to find out.