If it's gnarly; breaks, bad tweaks, face smack, head smack, etc. I'll go over to make sure they're gonna be okay. Otherwise, I try to help them laugh it off and roll with the punches. "Ow, that one fuckin sucked, huh?" or similar.
I busted my eyebrow at Burnside a few months ago by smashing my face into the flat, and got something of a standing ovation walking out to the street.
If it's more of a minor slam, you gotta take the power out of it and keep the sesh rolling