GURPSよろず

　Your ability is or seems sentient, and sometimes reacts poorly. Make an unmodified reaction roll whenever you wish to use it. For an attack, make this roll before each use. For other abilities, the advantage works until you need to make a success roll for it . . . then make a reaction roll. For instance, Flight requires a reaction roll when you try a DX roll to “push the envelope,” or an Aerobatics or Flight skill roll, while Dark Vision calls for a reaction roll anytime you attempt a Vision roll in the dark. Even “passive” abilities sometimes require die rolls; see Extra Effort (p. 160), Defending with Powers (p. 167), and Stunts (p. 170).
　On a reaction of Neutral or better (10+), the ability works as expected and you can attempt your attack roll, DX roll, Sense roll, etc., as applicable.
A Very Good reaction (16-18) gives +1 to the ensuing roll; an Excellent reaction (19+) gives +2.
　On a reaction of Poor or worse (9 or less), the ability fails. If it wasn’t already active, it refuses to activate. If it was active, it shuts down. This doesn’t normally endanger you directly; for instance, Flight sets you down gently and Insubstantiality leaves you in open space. Of course, the sudden loss of DR in battle, Dark Vision in an unlit room full of traps, and so forth can endanger you indirectly.
　On a reaction of Very Bad or worse (3 or less), the ability does endanger you, or turns on you in some unpleasant way. Flight drops you like a stone, Insubstantiality leaves you stranded inside a wall, your attack blasts you, and so forth. The GM should be creative!
　You can try to invoke a failed ability every second if you wish, but the reaction roll is at a cumulative -1 per repeated attempt as your ability (or the forces behind it) become increasingly annoyed by your requests. To eliminate this penalty, you must go for a full hour without using your ability.
Fickle is often part of a power modifier; see Fickle Forces (p. 24). In this case, the outcome of the reaction roll and the hour needed to lift penalties for repeated attempts apply to the entire power. The abilities of such powers can’t take this limitation - they already have it.

The GM may permit a weaker limitation: