I'm forever putting my foot in it completely intentionally. My brain just fails to engage before my mouth half the time. I could add a long list here but I'll share my, by far, most embarrassing 'foot-in-mouth' moment.
In my old job (close knit village shop) someone came in complaining how cold they were. To which I replied 'Oh, you need to be spit roasted' - which was met by such awkwardness & then I realised exactly what I'd said. For some reason before saying it out loud I'd only conjured up images of being twisted round a log fire. Cosy & warm.... Also found trying to explain yourself just makes it all so much worse! 
Anyone else got a funny or embarrassing mouth-before-brain moment?