Conan O'Brien made a triumphant return to the Tonight Show yesterday, after a freak accident during Friday's taping left him with a concussion and forced him to abort that night's episode. Thankfully, he's okay now, and doctors say he sustained no permanent damage. But a video of the incident — which aired last night, both at full speed and in slow motion — shows he's pretty lucky. For some not completely explained reason, he was running a triathlon with guest Teri Hatcher, and as he crossed the finish line in the Tonight Show studio, he slipped, fell backward, and smacked his head hard on the floor. Afterward, his speech was slurred, and he says he was unsure what year it was. Head injuries are no laughing matter, so we post this video only in the hope of educating our readership on the dangers of aerobic exercise.