An article about a study on short-term (aka "working") memory in "smart" people. It seems that when performace matters, performance decreases in people with a high working-memory capacity.

...individuals with high working-memory capacity, which normally allows them to excel, crack under pressure and do worse on simple exams than when allowed to work with no constraints. Those with less capacity score low, too, but they tend not to be affected by pressure.

I always screw up when some one asks me to calculate the tip for them. If they don't ask, I can do it right the frist time.