I don't know why but I find it fascinating when devs show extremely high quality promotional material and then the final game ends up looking nothing like was advertised. Especially when it's blatantly prerendered. It's not as bad these days as it was in the past but it still exists. So this thread is for posting these occurrences for entertainment mainly. Any platform, any year, etc.
Probably the most famous case of this is the Killzone 2 trailer from E3 2005.
I also recently found out about this game's trailer from E3 2006, called Eight Days. The game was eventually cancelled but it had two short tech demos and two actual trailers. I think this might be my favorite example of this kind of stuff.
The worst stuff like this from what I've seen was around the launch of last gen consoles, but I haven't looked up much stuff from the gen(s) before that.