When I just don't care.
*I've written a character that is all caricature, no complexity.
*My character has catchphrases, signature gestures, and nothing else. They're "the _____one."
*My character only exists to demonstrate some clever language, trick, or idea of mine that I think readers MUST see, because they must know how clever I am.
*They are boring, boring, boring.
Once in a while, I do write a shallow character on purpose, as a device that moves the story forward, or illuminates an idea or theme. (Or I try. Or just so I can say "I meant to do that.") But for the most part, even if they don't have a lot of time "on stage", or are only briefly mentioned in passing, I have to do my best to make sure they really mean something to the story, to me, and to the reader, or it hurts. And it's boring, or irritating, or both.
And that would be the worst!