I remember spending several hours tinkering with the spell check to explain to it that "cowrie" is a perfectly acceptable spelling for the shell (it wanted the one with the "y")
I'm worse than you on the idioms, since I like translated versions of foreign ones. To me "even experts make mistakes" can NEVER express my feelings as well as Saru mo ki kara ochiru ,"Even monkeys fall out of trees" ( I like Kappa no kawa nagare ("Even a kappa can drown" even better, but I stopped using it, as it got tedious explaining to people what a kappa was."
Ad then there are all the ones I made up when I was writing one or more of my fantasy/sci-fi stories, which become so familiar to me from having done all the writing that I forget that I'm the one who created them, and so they have meaning only to me. Oftimes I've been talking and suddenly gotten a perplexed look, then realized that I said something like "don't hire a wolf to protect your chicken yard from the fox" (Don't try and solve a problem with a solution that is actually a bigger problem) or "a fox's master" (an eminence grise ) or "But who made the wind?" (who actually solved the problem)
