Short for "sanity check," an infamous aspect of the role playing game "Call of Cthulhu." When players' characters encounter something of unspeakable horror (as they are wont to do in a game based on the books of H.P. Lovecraft), they are forced to roll dice to determine if their characters maintain their sanity in the face of mind-dissolving terror. In gamer slang, to "fail a san check" means to have one's mental faculties break down in a skull-twisting scenario.
Jim failed his san check when he found that his cat had run rampant through his studio, knocking over cans of paint and leaving little footprints of color throughout the house.