This bug is found when flying the planes from the WW2 pack.
After several minutes of flying in SSP or SMP (on lan server) without visible strain on client or server, the plane hits an invisible wall and the player falls out of the plane. The player can walk, but then modeled as someone sitting down. After this, the server takes a huge performance hit with max CPU usage and overburdening the server chunkcache, mostly resulting in server or client crash.

No specific messages related to flan's mod to be found in forge error log.

This has changed, you no longer get kicked out of your plane, your plane just stops and so you have to accelerate up to speed again, seems to happen when reaching the max speed of a plane (often occurs in a mustang rotary).

Occasionally the player will actually fall out of the plane however the plane no longer has any speed so there's no noticeable issue with chunks unloading.

I've never had a server or client crash after this bug.

It would seem that the server just stops sending the player location or something because you can logout and then log back in to be where the plane is.

Seems like it's been a while and the bug still happens in 2.4.

Tested with the Civil Parts Content Pack. Seems to only happen in the Boeing 707. There the plane won't even take off and the player already has fallen out of the plane.