My three biggest gripes are:
Personal equipment/webbing that has been dismantled and is shown scattered about a vehicle or defensive position. As a former soldier, I cannot get my head around this. We were always taught to keep all our gear together, so if you got "bounced", you grabbed your kit and "re-grouped" later. No time to scoop up canteens, magazine pouches, rucksacks etc... Even on an AFV, our vital gear was usually attached to our web belts, even if we did not wear the full set...
Second gripe - artillery/tank rounds. My basic Gunnery training had a whole lesson on handling cartridge cases and shells. An artillery or tank cartridge case is usually fitted with a primer, same as any rifle/pistol bullet. This is quite a sensitive component and the vast majority of trained soldiers would be very careful about resting a cartridge case on it's base, just in case the primer came into contact with a sharp piece of gravel or a rough stone, the consequences of which could be noisily spectacular!
We were taught to always lay the cases out flat on a tarpaulin or poncho. I get somewhat annoyed when I see dioramas with cartridge cases leaning up against road wheels or other items or lying loose on an engine deck. Empty cases, yes, but not "live" rounds...
Finally, don't overdo the weathering! Most squaddies will look after their vehicles and equipment to some degree, if only to keep the Sergeant Major off their backs! There's nothing more annoying than a "serviceable" vehicle that looks as though it's leaking oil from every orifice and been stuck out on a gunnery range - as a target - for two or three decades!