Definition of Artifacts from the web:
"Unwanted visible effects in the picture created by disturbances in the transmission or image processing, such as 'edge crawl' or 'hanging dots' in analog pictures, or 'pixelation' in digital pictures."
For me, when I was OC'ing, I would start up Oblivion and start to see green/red/yellow polygons rapidly flashing in characters' faces and in other part of the world. This was when I quickly exited the game and lowered my clock speeds.