Okay, now that a Glitch thread exists, here is what I found playing GTA IV.
Of course, any GTA fan knows about the ghost swings at the projects at Firefly Island. Drive a car into them and get launched halfway across the city. But no one seems to have found what I call the Kate Date glitch. Probably because no one wants to date Kate. She won't invite you into her house for sex, and her brothers promised to kill you if you ever really did. You can't unlock any ability by dating her, so why bother when there is so much other mischief to do. But if you ever do decide to date this prude, first steal yourself a soda delivery truck. I was on my way to Kate when I flipped the car I was in, and had to improvise by carjacking a soda delivery truck. Kate works at some sort of community theater. Use the GPS to plot the route to it. Drive into the yellow arrow at full speed. When the cut scene is activated, the truck will drop from the sky and land on top of the marquee. Of course Kate will complain about where you chose to park the vehicle, and will yell at you when you drive off the side and down to the sidewalk below. This only seems to work with trucks, and so far only the trucks with the soda logos painted on the sides land squarely on the marquee and not off to the side.
(https://i.imgur.com/Mk2d6pF.jpg)