In GURPS, the flipside of critical successes are critical failures, or misses in character combat. Like critical successes, critical failures mean an extreme result from a roll — in this case, a bad result, but not necessarily the end of the world. A roll of an 18 is always a critical failure, and a roll of a 17 is a critical failure as well if the skill in question is 15 or less. A roll of 10 over skill is also a critical failure.

Roll Effect
3, 4 Your weapon breaks and is useless. Exception: Certain weapons are resistant to breakage. These include solid crushing weapons (maces, flails, mauls, metal bars, etc.); magic weapons; firearms (other than wheellocks, guided missiles, and beam weapons); and fine and very fine weapons of all kinds. If you have a weapon like that, roll again. Only if you get a “broken weapon” result a second time does the weapon really break. If you get any other result, you drop the weapon instead
5 You manage to hit yourself in the arm or leg (50% chance each way). Exception: If making an impaling or piercing melee attack, or any kind of ranged attack, roll again. If you get a “hit yourself” result a second time, use that result — half or full damage, as the case may be. If you get something other than “hit yourself,” use that result.
6 Same as 5, but half damage only.
7 You lose your balance. You can do nothing else (not even a free action) until your next turn, and all your active defenses are at –2 until then.
8 The weapon turns in your hand. You must take an extra Ready maneuver before you can use it again.
9, 10, 11 You drop the weapon. Exception: A cheap weapon breaks; see 3.
12 Same as 8.
13 Same as 7.
14 If making a swinging melee attack, your weapon flies 1d yards from your hand –50% chance straight forward or straight back. Anyone on the target spot must make a DX roll or take half damage from the falling weapon! If making a thrusting melee attack or any kind of ranged attack, or parrying, you simply drop the weapon, as in 9.
15 You strain your shoulder! Your weapon arm is “crippled.” You do not have to drop your weapon, but you cannot use it, either to attack or defend, for 30 minutes.
16 You fall down! If making a ranged attack, see 7 instead.
17, 18 Your weapon breaks; see 3.