The number of times I've designed & re-designed a part to fit, an assembly to fit.. (Gotta love a good CAD program & a 3D printer!!)
the few times we get called into the bosses office because a drawing was off, and the 150 parts that came in were scrapped..
The few times, with the entire eng department signing off on a design, and the $$$K in prototype parts & tooling come in, only to find we overlooked a small, but important aspect: How do you get your hand or tool in there to assemble / fix it?!
The few times, getting specs out of a catalog, and they're wrong / off, or just not there.. or spec'ing a part in, only to find out it's no longer available..
Expecting ±0.010" on an assembly over 6' and with 17 pieces..