The classic example is the Cadillac Cimarron, basically a dressed up Chevrolet Cavalier, and it was a dismal failure. GM seems to have learned their lesson, at least partly, as their shared models are a lot more distinct than they used to be. But sometimes, I think the car genes just won't allow you to hid the fact that two cars are close relatives.
Pontiac's Aztek is like the Quasimodo of the automotive world, a horribly disfigured model that blemishes a marque that usually has some of the best looking models in GM's lineup. The Aztek's mechanicals are also the underpinning of the Buick Rendezvous, and somehow Buick got the styling right, well almost. From the B-Pillar forward it's a sleek, modern, and graceful design. Pleasing to the eye, it embodies the typical Buick flair, until you get to the C-Pillar.
Everything then goes wrong, and it looks like one designer did the front, while a less skilled subordinate finished the rear. It's the Aztek gene, it has to be. It reared it's ugly head and gave the Rendezvous a hump on it's back, just like it's brother's. At least it spared the face this time.