Random Effects Roulette

People often want abilities that produce random effects when activated but GURPS doesn't have a published modifier that does this. 

Roulette -20%

Your ability produces a random effect from a list. Every ability on this list must have this limitation. Pay full price for the most expensive ability and take the rest at ×1/5 cost as Alternate Abilities or Alternative Attacks. Make a table with an equal chance of each ability, and roll when you try to activate any of them, the resulting ability then activates.

If all of the abilities are attacks, then this is a transient ability that activates with an attack maneuver. Otherwise it's a switchable ability that requires a Ready, but whichever ability you roll is active (or available, for transient abilities) for one minute (unless it would ordinarily have a shorter duration).

This is based on Fickle, GURPS Powers p, 110 and essentially works by replacing the Reaction table with another table. 

The reason why it is not based on Accessibility, (e.g. if you have four abilities that each work 1/4 of the time -30%), is because these kinds of Accessibilities are priced based on the ability either working or nothing happening. A random ability that always does one of 100 things isn't fairly priced the same as an ability that only works 1% of the time and doesn't do anything at all 99% of the time.