I own up to it, immediately. Much better than a big "blamestorming" meeting. God I hate those.
I got called to the carpet once by a "situation" that got escalated. Basically I told the guy I was talking to that he was a moron and it's not my job to educate him and if he would like to stay on the phone I will educate him. Big meeting with my VP and I just said "My bad, I just lost my cool". VP had a ton of respect for me from there on out, he expected me to blame somebody or something outside of my control. Nope, I just lost my cool and it won't happen again.