Repairs that won't last, or mods are trendy. Both create extra work for the next owner years later.

Dash and door panel hackery for aftermarket radios and speakers. See that a lot and many times the radio is low end junk.

Shaving drip rails and door handles on old cars.

Thick application of Bondo instead of lead where lead is the most appropriate material.

Cheap window tint that turns purple and is full of air bubbles. If it looks bad remove it and redo it at least.

Cutting a hole in a hood for a round chrome air cleaner that sticks up 1/2" above the hood. At least get a scoop for it. Or locate a drop base air cleaner first to see if that could have been avoided.