Just thought this could make a fun topic...
I'm terrible for taking people literally, I would say my most common response to conversations between people at work is "are you being serious?" the other day my mum had a right laugh when we were chatting about me putting money into an account for my dad because I'm living at his house and he needed to use it on his car repair bill so she said well at least he had it there for a rainy day and now it's definitely raining. I looked outside and said but it's not raining, it's sunny out she meant metaphorically and then I realised after all these year that a "rainy day fund" meant for when things were bad. I never realised that. I remember when I first heard the term and I was little and I remember being bored whenever it was raining so pictured it as saving up to do something fun when's it's raining and your bored, as I got older it never occurred to me to think about it's meaning or to question that explanation... Until now, my mum thought it was funny at least.
I guess there must be plenty of funny stories out there where idioms and metaphors just haven't worked.