I've noticed cars are jumping a little sideways when watching replays and I'm wondering if anything can be done about it and if it's a known bug of the LFS replaysystem?

If it's a mutiplayer replay that's because of the small amount of lag that's in common with online gaming.

Okay then I understand where it comes from. Thanks for the reply.

Also because a MPR saves the position of the car and its characteristices (speed, and forces), while a SPR "plays the game" on its own (the SPR saves the inputs, and the physics engine make the car performs as if someone was driving).
And dependind on the packet's frequency, you can have some weird things happening in MPR, such as a car getting body damage before a collision, or your car sliding/drifting here while you did not do it online.