It's got to be the opaque coolant tanks (yes, I drive a 1.8!), in a wondrous choice of black or magnolia. A critical part of the machine that can only be opened when cool, and somebody thought "lets make it impossible to check after a drive"
To be fair, the reason for that is probably the internal construction of it needing a harder plaster but it's still very annoying. I've been tempted on more than one occasion to fit an aftermarket bottle to see what happens but have more interesting jobs to work on for now.