AI crews or pilots are abandoning their vehicles/choppers if you switch to another playable group. This was fixed in Arma 2.
This issue pops up only if you start the game with crewmen or pilots on foot and ordering them into empty vehicles.
Imo this is serious because it messes with gameplay in a big way, what's the point of having multiple playable groups if the previous group always abandons their vehicle.
Same thing happens with infantry ordered to enter a vehicle as passengers, as soon as you switch to another group they leave the vehicle.