So I’m in multi-player driving down Prague bridge going straight, and everything seem to be going well when all of a sudden my car just out of the blue makes a hard left without input from me. Is this some kind of lag situation or is there a cheat or mod that allows others to manipulate your car? I’ve also been in races where it seems my car gets a massive jolt of acceleration at inappropriate times. Or am I just going slightly mad cue the queen music.
Lag does indeed cause some weird stuff like that.
There have also been controller issues for some people. (Check the batteries) I don’t know much about the controller issues, but there have been various reports of both battery and non battery related issues.
Either way, I doubt it’s a mental issue, but you never know. 
I sometimes have my car decide to pull a HARD right or left on Road atlanta and Prague under hard braking
tires remain straight suddenly WOOP into the grass I go
This is very possible to have happen with damage to the car, and might have nothing to do with lag.
I was not breaking, just going down a straight, only half way over the bridge. it was strange
Of course you’re crazy. You bought Forza, didn’t you?
This answers so many things about me…
i had similar problems with my day one controller, which made playing forza un-playable it seemed.
tried a friends controller and it was fine. returned to my controller and it was bonkers again. grrr
so it may just be your controller
I know when my controller is about to go dead it starts acting all funky and doing things I don’t want it to do, which is why I charge it every night now with the play and charge kit.
Also, on Xbox One, make sure you do the Controller Update if you have not done so since receiving the latest Xbox LIVE update a few days back. It is in Settings/System at the bottom of the list “Update controller.” That could make a difference, since it changed in the latest release.
After you’re sure you have updated the controller, in FM5 check to make sure any personalized settings you may have had are where you want them. That’s the in-game settings.