Girlfriend GlitchEdit
If you blow your girlfriend up with several radio-controlled bombs, she will not die.
Monster GlitchEdit
Obtain a Monster and park ONE wheel onto a random car (sideways or front on, both work). Get out of the Monster, get in the car you park on, and you may find yourself spinning 5000ft in the air, crashing back down to earth.
Taxi Driver Mission GlitchEdit
A glitch can occur when the player begins the Taxi Driver mission, but no fares are available. It happens when the player has crossed over the map boundary, which can corrupt save files. It is advised to do this mission very early to avoid this glitch.
End of the Line Mission GlitchEdit
After completing the mission End of the Line, all law enforcement vehicles will disappear from the players garage (police cars, Rhinos, and even the SWAT Tank). This may be a glitch, but it is most likely to stop players from obtaining the SWAT Tank (as End of the Line is the only mission to obtain it).

A Freight passing through the Brown Streak hijacked during the mission End of the Line.
Added by Megatronacepticon
End of the Line Mission Glitch 2Edit
During the mission End of the Line, the player will encounter an engine carriage of a Brown Streak. Unlike most trains that appear during missions, this one can be hijacked (with the penalty of losing Sweet, thus failing the mission). If this train is jacked, other trains will appear on the player's track, passing through the player's train.
Ghost bike GlitchEdit
If you ram into a bike with a driver, and then quickly drive away, whilst looking back the ped disappears, and the bike will drive with no one on it. This happens if you are far enough away that the ped disappears whislt they are doing the 'get on' animation. The bike reads that the driver is getting on, and drives like normal. But the ped is gone. The bike will drive normal until its flipped over or jacked. The player will not punch anyone off if you try to steal it. Video:
Interior GlitchEdit
If you go into an interior, and have a parachute on, switch to it, and the altitude meter will be all the way at the top, because you're in the hidden interiors universe.
Flying Train GlitchEdit
In the mission Snail Trail if the player follows one of the Freight trains that spawns instead of the Brown Streak the Freight will make a right turn off the tracks at Cranberry Station and fly across the map until reaching the tracks at Red County where the train will make another right back onto the tracks and continue on its journey like a normal train.
This is caused by the path file within the game only being coded as far as Cranberry Station. When the train reaches the end of the path, it heads directly for the next set of coordinates, which are located in Red County, way accross the map