I get positively flustered on the rare times I have ever actually swore in front of my Mom.
I tend to employ profanity with friends, but usually only if I'm upset (either by said friend or on said friends behalf). The most common swear I usually let loose with is "damn it!" or "god damn it" (despite my militant atheism, and my one swear I abuse when I'm frustrated at, say, one of our roommate's ancient cats climbing atop our dryer and shitting all over it it "god-fucking-damn-it" said at the same time as a frustrated sigh.
I also have a superstition of sorts carried over from my Presbyterian days: never, EVER swear in the presence of a minister, ever.
So last year, when I was helping a friend assemble heavy metal scaffolding in the backyard of his Dad -- his preacher Dad's, and the entire weight and point came down upon my foot, my immediate instinct to swear was overridden in my lizard-brain: DON'T SWEAR. ESPECIALLY DO NOT SAY 'GOD-FUCKING-DAMN-IT!
So instead, apparently, my brain figured plain old vulgarity was better, because I let loose with a howl of pain and "Ow! Cum-guzzling son of a WHORE!"