Or if a character is insufferably drab even for a cameo, so you assign a trait. He's bad-tempered, he's mischievous, he's lazy -- and the next thing you know, he's looking for new locations in the book to be bad-tempered, mischievous, lazy. . . .
Once I realized that a character had taken on too much importance in the first chapter. So I killed him in the second. Didn't get rid of him. Dead as he was -- and stayed -- he came up again and again during the novel.