production mistakes - In which scene is the White Van in Braveheart?

It's a bit different to Seanland's answer, but this site gives a couple of instances (both on the first page).

Visible crew/equipment: At the funeral of William Wallace's wife, Murron, a white van can be seen. He bends down to kiss her and as he stands back up, if you look over his left shoulder through the trees you can see the van going past. It's very quick, small, and blurred, but it's there.

It's not during a battle scene like you asked, but (apparently) it is van and not a car.

Revealing: When Wallace walks up to the man who killed Murron and faces him. Look at the man behind Wallace and the man. You can actually see a car passing behind his head in the distance.

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.