Best as I can tell, RDA actually did get smacked in the face (or maybe he had a mole removed surgically?). It seems to have left a genuine scar on his eyebrow that lingered for some time and was a lot more visible with the eyebrows going grey, once the business of trying to dye his hair Young ended. So they wrote it into the script (Demons, IIRC) and stopped covering it with make-up. By the time they were doing Wormhole X-Treme, they were sending it up -- the guy in that had multiple scars through his eyebrow.

Reminiscent of when he broke his hand during the first season of MacGyver: they shot around the cast for The Enemy Within, then wrote the damage in as two separate whumpages for To Be a Man and Every Time She Smiles. And tried to make him agree in writing not to do hazardous things any more . . . which Did Not Work.