One trap which I was recently shown on my own vehicle is the drivers side inner wing (structural MOT failure). What was worse is while there was a small amount of bubbling of the paintwork it didn't actually look too bad (and the underseal was all nice and clean looking), but it had actually rotted through in one area, so that the screwdriver went through with no effort whatsoever. I'm told it's a very common area and the first place that most VW mechanics will poke, the passenger side inner wing rarely gets such bad rot.
Also the bottom corners of my driver doors (think there were drainage points there that got blocked up) have become rusted so much that you can use your finger to happily poke through.