Maybe not a slip-up, but a last-minutes changes (so I heard) that didn't get 100% corrected.

In an early episode (can't remember which one), as Homer is leaving for work, Marge stands at the door, puckered up and waiting for a kiss good-bye. In the original version, Homer ignores Marge, and opens the door in her face, leaving a lip-stick mark on the door.

This bit was cut out of the version that aired, but through the rest of the episode, the lip-stick mark can be seen on the door.