Originally posted by tasuki
I think basically any two words involving a, o, and u... At least that seems to have been the ones my students (back in the days when I taught English) had the most trouble with...
Aren't the katakana spellings even the same for some of these "u" and "a" words? I'm sure there are others, these are the ones that come to mind, though.

ラック-- Lack, Rack, Luck, Ruck
トラック -- Track and Truck