When Sam arrived home after dropping off the painting and Endora was there, Tabitha runs off towards the kitchen and Sam rushes after to get her while still saying her lines. Erin Murphy says her running off was not part of the script and Elizabeth Montgomery saved the take by catching her while still continuing her lines.
There actually was an American artist named Elizabeth Montgomery (1902-1993) whose paintings were displayed and offered for auction multiple times, but is of no relation to the actress.
The title comes from the expression "art for art's sake" expressing a philosophy that the intrinsic value of art, and the only "true" art, is divorced from any didactic, moral or utilitarian function. A Latin version of this phrase, "ars gratia artis", is used as a motto by Metro-Goldwyn-Mayer and appears in the circle around the roaring head of Leo the Lion in its motion picture logo.