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.
In German we use the English term 'understatement' in some occasions, e.g. 'his dressing style is a clear understatement...'. My brain somehow tied the meaning to something like 'understanding', maybe due to the similarities of both words. For decades it was clear to me that someone dressing like that were dressed to the point and 'making a clear statement'. Now that I've checked the real meaning, I'm completely puzzled when and how to use the term and what I've misinterpreted all the years...