I can never remember which is which:
both are popular seasonal fruits in Korea, one you eat the seed and the other you eat the flesh, and I've spotted and figured out which is which a dozen times, only to forget (like when I was seven and I couldn't figure out left from right until I figured out the "Left hand forefinger and thumb makes a capital "L" shape" rule of (haha) thumb. I didn't really get it nailed down until I started learning to drive.)
I also used to always mix up the words prostate and prostrate. The highlight of that confusion was the time in my Fantasy Literature 300 level class in university, when I said "The priest fell, prostate on the floor" and I got a good snicker from everyone, and even a snarky little comment from Dr. K.
what two words/things do YOU mix up, however many times you've tried to remember? Ever embarrassed yourself?