This video has been around for a while, so you may have seen it already. And if you have, that's awesome, you should definitely mention it the comments section. I saw it some time ago myself, but not with this guy's voice-over, which made it better. It's probably NSFW though, since every other word the dude says is a curse word. Oh, and it's unbearable long, so let it download and then just skip around for about 30 seconds or a minute (depending on how you feel) to get the gist. If you like it there are about a million other videos on Youtube of hacked Mario levels that are unbelievably impossible (search mario impossible or mario frustration and feel free to post links to any good ones). While it does look tempting to give playing one a shot, I know exactly how it'll end -- with my leg stuck in the television and bleeding (yes, I had to buy an old CRT from the thriftstore after my ladyfriend traded the LCD for "the most comfortable pair of shoes ever").
Thanks to Randomnigel, whose blog I found this on, for having it there