Every ship involving characters who have no interaction and/or chemistry.
Bluestar x Thistleclaw is such sin. Only a villain would ship that.
Darkstripe x Tigerstar is very upsetting. Really any ship between a villain is pretty nasty. Most all villains have the same manipulative trait. They use other cats and abusive towards their loved ones and the ones around them. Not good ship material my dudes.