All those interminable forms we have to fill in. Or surveys. Or feedback. And then the data at the end so that they can analyse who responds to them etc. etc. Age. Ethnicity. Gender. Etc., etc.
So this one asked 'What is your gender?', two boxes, Male and Female. I'm in a bad mood anyway, so in the 'Any other comments?' box I asked that they reword the question to 'What is your sex?'.
No wonder people conflate the two words all the time. The words are constantly presented to people as synonyms.