A Raven MTi2, which is 1080p on HDMI, and an LG 34U97, which is 3440x1440 on Mini DisplayPort. It works sometimes, but after an hour or so the LG just says "Out of Range". Sometimes with them both plugging in, the Raven gets a messed up display while the LG is blank. Either one works just fine on its own. I've tried replacing the cable several times, making sure in particular that the MiniDP one is rated for that resolution.
Answers like this suggest it's not possible, but why does it work sometimes?