well, apparently they don't cause any trouble whatsoever. they have been doing studies that the "anal leakage" is, in like 9,999 of 10,000 people, entirely psychosomatic (and the 1 is an allergy, which can be triggered by a lot of other foods, too). basically, in order to fuel late-night-talk-show jokes, and for non-olestra-using companies to push their merchandise, the "anal leakage" thing was publicized and led to mcdonalds to drop their (very-far-along in the implementation process) plan to replace their fry oil with a mixture of olestra and real fat. the article extrapolated with heart disease figures and that move would've saved THOUSANDS of lives.
but oh well. it was funnier to make fun of anal leakage whether it existed or not than to have a bunch of fat people alive.