I just found a wall at the Vinewood Bowl that you can jump through and proceed to skydive underneath the ground texture, seeing the shells of the buildings and cars above you. You can still shoot but it seems to just hit the invisible ground texture above.

I'll post a video of it later if I remember.

Anyone else find any cool glitches out there?

Put your car in front of a closing fence and it will get launched in the air.

http://www.reddit.com/r/GrandTheftAutoV/comments/1n7jrs/my_gf_found_an_invisible_ladder_last_night/

Invisible ladder.

here's the spot where you can dive through to the other side...

I forgot to mention in the video that it only spawns the parachute once per play, apparently. Everytime I'd try to do this after the first the parachute wouldn't spawn, but after reloading my save and going back to this spot, it did.