In basic GURPS, a caster's skill is considered to be no higher than sixteen for purposes of margins of success for resistance rolls. So if Rincewind uses his Daze-25 spell to stun Carrot (Will 12), there's only a difference of 4 in the Quick Contest of Skills (16 and 12) instead of 13 (25 and 12). This breaks down when characters really can get stats higher than other people's skills: if Rincewind uses his Illusion-20 against Steven's IQ 20, Steven has an edge, because Rincewind's Illusion is knocked down to 16. This exposes one of the larger flaws of GURPS: stats and skills are only reliable at 12+, but can't really be meaningfully taken above 16.