Not taking time to eliminate the oil pan/transmission seam. I've seen some really nice models on here, built very realistically, but when you get to the underside pic, theres that seam!
Plug wires that seem to defy gravity by shooting straight up out of the distributor and making a giant arc on their way to the plugs. I call this the "spider on the engine" syndrome, and the model would actually look better without any wiring at all.
Another pet-peeve I have is seat belts, especially race cars. I have never seen a real race car where the driver got out, reached back in, routed the harness neatly over the back of the seat and the lap belts neatly over the side, and buckled them in the center. I'm not saying it couldn't happen, I'm just saying I've never seen it (been around a lot of race cars) and it's not likely. All of them I've seen are dangling over the sides of the seat, so the driver can easily get in without having to move them.
Same deal with a 70's "period correct" street machine.....back in those days, the seatbelts were either shoved down in the crack, pretty much out of sight, or removed completley....NOT neatly stretched out across the seat. The only time you see that is at modern car shows or cruise ins.