This is not a glitch as such but a clever technicality. What you do is to enter 
the training mission in the one player mode. Then, play through until you can 
select the helicopter and fly it to the sugarlumps on top of the jar of chocolate 
on the table. Pick one up and carry it to one of the gravity-changing ramps at 
the wall and drop it right at the top of the ramp. If you have it right, the 
lump will change gravity to stick to the wall. 

This alone is quite amusing, but then for something extra, you pick it up again
with the helicopter and fly away from the wall. The gravity of the sugar lump is 
still towards the wall so whenever you let it go, instead of dropping to the floor, 
it will fly across the room like a missile and stick to the wall again! 
For something extra, with the car try to push the toast up the ramp as well. You 
can do this with any object, but with the toast, shoot it a few times to make it 
burst into flames. Then pull back a bit to observe: A piece of toast flaming while 
being stuck to the wall.