Whenever this happens I say things like "What, you never met someone with brain trauma caused by a drunk driver before??" Or (because I'm old enough) "Yeah they never were able to get all the shrapnel out of my head after the grenade incident in Iraq. " They immediately feel mortified (which takes the pressure off me) and start apologizing profusely. Then I'll be like "nah I'm just playin. I just have a stutter. " Works out well when everyone is self conscious for a second, not just me.