Another entry in an irregular series.
skirl and swirl. Possibly because it sounds a little like swirl and a little like skirt, a lot of people apparently think skirl means roughly the same thing as swirl. But skirl actually specifically means the sound that the chanter of a bagpipe makes. To skirl is to emit such a sound; to skirl a tune is to play it on a bagpipe.
While I'm here, another word that I always used to be unclear on the meaning of: erstwhile. It doesn't mean the same thing as esteemed; it means former. This is a word that I picked up from context; I assumed that "my erstwhile colleague" (the phrase I usually heard the word in) meant "my esteemed colleague," but it really means "my former colleague." I wasn't alone in this misunderstanding; many people misuse this word.