When you get in a helicopter, you have to wait for the animation to end before you can use it. With some helicopters, you can use it while the animation is still occuring.
Bugged Helicopters:

• WY-55 Hellcat (Green)
• WY-55 Hellcat
• PO-30 Orca (Black)
• PO-30 Orca
• AH-99 Blackfoot {F22894}