I've heard many rumours of the infamous movie mistake - the white van, visible in the background of one of the battle scenes in Braveheart. However I've never spotted it. Anyone have a timecode or key identifying moment and quadrant of screen to look for the van?
It's a bit different to Seanland's answer, but this site gives a couple of instances (both on the first page).
It's not during a battle scene like you asked, but (apparently) it is van and not a car.
I don't have any timecodes or video links to check the accuracy of these things (or the site in general) but you should be able to find the scenes from the descriptions.