This generally doesn't happen to me. I don't buy my games new. I have decades of older, cheaper games to catch up on. The newest game I own (that I've actually gotten to start up) is [Prototype], and it's been fun.
However, I can remember one thing from a few years ago. Bionicle Heroes. I remember seeing that as a kid, and wanting it so badly! I watched that 10-minute gameplay video over and over and over again. I was so excited. Then, I got to play it with a friend of mine on his X-Box 360, and I was like "What the heck!? This is the worst game I have ever played! How come they downgraded so much!? Why are there so many inconsistencies with the actual Bionicle cannon!? Why does Matoro (I forget the spelling, white Toa dude) have the Sniper scope, when it should be Heuwki (Stone Toa, may have spelled his name wrong)!? Why are there vizorak and borak, and why are they so small, and why are all these ancient characters here, and why is Fenrak so huge, and why does shooting him hurt him now, and why did they remove all those cool sounds, and why isn't there a death penalty, and why are the Lego pieces all wrong, and where are these weapons coming from, with no Zamor Spheres, and why am I fighting Axon, and why are the characters able to rip their heads off and throw them around, and what the heck is up with this Piraka Playground, and...!?
Yeah. Needless to say, I was severely ticked off. That was pretty much the end of my interest in the Bionicle franchise, as well. I even remember this one guy in the development team asking for questions from the fans, and I asked him about all the ridiculous paradoxes, plotholes, and inconsistencies (using an actual Lego font, as I recall), as I was very well brushed-up on Bionicle cannon, being very ticked off with how bad they messed up countless aspects. I don't think he responded. I was wondering why these people would make a videogame based on something the obviously had extremely insufficient knowledge about. That was when I was a kid. Now, I am much older, and I see this (inexcusably) happens all the time. Hello, The Last Airbender.