Just recently I was in a conversation with a number of UK mainlanders and we had a debate over what "tories" meant, apparently disproportionately ordinarily it refers to a political party and it's not usual to use it as short for "territories" as I've used it (according to how the debate ended, it was half and half between them). And once again I'm reminded of how people feel to look back at their usage of a word/phrase over the years and cringe.
More tragically, me and a friend were embarrassed once upon realizing everyone was confusing "encephalitis" with "hydrocephalus" when talking to someone about their kid with hydrocephalus. Awkward because encephalitis is caused by HIV.
Not a word but I thought the idiom toe the line meant basically the same as push the envelope. As in you're testing the boundaries of something by walking right up to the line and nudging it with your toe to move it further.
Turns out it means pretty much the opposite, essentially the same as fall in line.