VW Passat 4cyl. PCV tube broken under the intake manifold and broken dip stick tubes on Jettas and Beetles.
Larger than stock exhaust before the cat on diesels. 7.3L Ford, Chevy, and 98 Dodge have 3" stock. 6.0L Ford is 3"-3.5" at the flange, and 03 and later Dodge is 4"
EGR cooler missing on air cooled VW bugs and buses. This is the Square box underneath by the muffler.
Missing "thermo reactors" on early Porsche 911s and 930s. We don't see too many of these but they are always missing the thermo-reactors and some have catalytic converters installed to pass the tail pipe readings.
Good judgement comes from experience. And experience? Well that comes from poor judgement.