There is a bug that causes you to automatically mount a vehicle when right-clicking with the vehicle directly behind you. This is happening on my multiplayer server.

The bug still exist in version 0.20. It only happens when you are looking up, while right-clicking with the vehicle directly behind you.

Pinging @Phylogeny for this as they know the raytracing system the best.

It actually happens regardless of looking direction. Turns out I screwed up the triangle raytrace algorithm; it was actually doing a trace of the infinite line parallel to the player's eye-centered looking ray. It now actually traces the ray.