I'd have to elaborate here.

First off, I can't reproduce it for the life of me, nor can I confirm that it happens with any vehicle. It happened randomly. What happened exactly is that I tried boarding a Black Hawk for probably the 20th time when the screen went black (IIRC HUD was still visible). Nothing occurred for about 4-5 seconds and nothing I did seemed to do anything. After reloading the world, I found myself in the void. Had to MCEdit my way out of there in order to avoid death.

When I got back to where I boarded the helicopter, I noticed a shadow on the ground. WAILA showed it as a wheel (it was invisible). That was the only sign of a heli ever being there. Turns out that reloading the world while in a vehicle removes the vehicle from existence, but that may be another bug. Didn't teleport me to the void when I tested that one.

I'm aware of the issue and have been able to reproduce it. Unfortunately I've been unable to track down the cause so far.

The issue occurs with vehicles that have existed through the vehicle being saved and reloaded e.g. closing single player and re-openings.

I had been told that this was fixed... Nope, the problem is still there. Might be cause of other mods, maybe... Or perhaps overall lag from the amount of things going on around.

Extra info: When boarding, the void is visible for a moment (in my case, as I don't crash) and then everything's fine, but for a friend, he gets stuck on the void part and crashes out.
Also I think I recall some weird things with.. wheels. Like, the server console outputting that some wheels had to be deleted because of problems. Or maybe it was seats.