For two years before I realized my mistake, every day when I reminded my in-laws to do their daily exercise (zuò cāo / 做操) I said the second word with a 4th (falling) tone instead of a 1st (high) tone, thus telling them not to exercise (操) but to fuck (肏). Oops!
(Fortunately they are too cultured to hear it that way and/or too polite to correct me.)