I haven't told anyone this in RL because I am actually mortified at myself. I am a fluent, but not native, Spanish speaker. But for some bollocky reason I said something completely fucking stupid recently.
Instead of asking for a vanilla ice cream cone I asked for a vanilla cunt. 
What kind of idiot am I? To make matters worse, the person serving me laughed with her colleague about it (at me, not with me!) I was too mortified to laugh at myself and haven't stopped cringing.
Help me get over it by telling me some gaffes you have made, either linguist or just plain stupid.