I have a couple.
First one is a bit of a story. My high school has this little media room that is barely supervised where students figured was a good place to get high. But my friends and I dragged a Gamecube and original Xbox up there and played a lot of smash and Halo 2. Problem is that they were my consoles and I didn't exactly get all the parts back so now I got an Xbox and Gamecube without the cables to run them. I'm sure I can get the parts online but eh, the Wii and 360 are backwards compatible so I'm not really in a hurry. Still, having these big chunks of plastic lying around isn't great either.
Second, learning the plot of Spec Ops: The Line before playing it. Extra Credits seemed quite taken with it so I completely ignored they're spoiler warnings thinking the twists can't be that great and then they totally were. Gosh darn it.
Oh, I have one more.
Not being smart enough to understand Bioshock Infinite's ending. I'm a guy who likes to be spoon-fed information, call me dumb, that's just how I take my entertainment, easy to understand, I don't think too much when partaking in entertainment, so Infinite's ending really threw me for a loop. The main reason I regret is is because as I slowly came to understand it, the revelations slowly revealed themselves, which would usually be fine when telling a story but the ending of Infinite is clearly meant to be one big punch. I had all this information not understanding it and as I slowly *started* to understand it, I was re-learning it without any punch. I tried freaking meditation to try to get my mind in a place that could feel the weight of the ending properly and I almost got there but it didn't completely take. Still, I think it's a pretty great ending even if I didn't feel it as much as I could have.