Landing a helicopter on the Helipad at the hospital at Kavala, the helicopter vibrates up and down (bounces), even with the engine turned off. When the menu is open, the option for getting out of the helicopter blinks very rapidly, as it turns on every time the helicopter is down when bouncing.
EDIT: After further testing, only the Ghosthawk has this problem. So it may be an issue with the Ghosthawk, and not the helipad.
EDIT 2: After landing on a pier, the Ghosthawk bounced as well. This has led me to the conclusion that the geometry of the Ghosthawk somehow interferes with certain terrain types.