When you have a flashlight in your pants and in your hands and you have it on then you change to a new pair of pants on the ground, the flashlight effect radiates from the pair of pants you took off (with the flashlight in them).
Say flashlight was equipped to slot 1 - it appears on but has ghost slot (non-inventory) image, however you are still holding it, AND it is shining up on the ceiling from the old pants.
Press quickslot to drop the flashlight, then drag from old pants to new, and everything is A-OK. Still a strange bug... can't drag from old pants to new without first putting it down (when its already shining on the ceiling from the old pants) {F28580}