Over at bit-player, Brian Hayes has a tale of woe about implementing a geometric algorithm. Not even a complicated one at that, merely an algorithm to check if two line segments intersect. His story is the usual one: general position is a fiction thought up by sadistic geometers (but of course !), and even the simplest of ideas needs many special cases to take of degenerate data.
Apart from being surprised that he didn't have problems with precision (how DO you tell if both endpoints of a line segment are identical), I sympathize with his plight. As anyone who's ever implemented a geometric algorithm will tell you, it's much harder than it seems, and to an extent, the idealized algorithms geometers design can be hard to implement. CGAL and LEDA have made life a lot easier, but robustness and exact computation are still important (if highly uncool) areas of computational geometry.