I noticed this small visual oddity in an elevator instance I was making, and as far as I can tell it's in every Valve elevator and probably most custom versions. (Screenshot from sp_a2_triple_laser):

Just below the crosshair there are two black ridges, which repeat around the tube cover 3 times. The model is basically aligned at right angles, but it seems to need to rotate to align with the three rails. Nothing else on the tube models align with it. When it is aligned the ridges fit perfectly around the rails. In the elevator logic instances, the cover is aligned "0 90 0", but it should probably be "0 120 0".

I'm just wondering if anyone has an explanation for this.

http://m.memegenerator.net/instance/33032616

portal 2 spoilers inbound as if you haven't actually beaten portal 2 yet

It's probably something small valve skipped over when making portal. If you look closely there's many things that don't make sense. For instance, in the chapter The Fall, as soon as you exit the elevator shaft, look to the far left where the giant rock wall- oh wait, it's just a clearly visible skybox. Also in the same map, the vault alarm light does not have a source. If you look up during when Wheatley breaks you out, right before the hallway before GLaDOS smashes the chambers together, the vactubes are awkwardly going into the walls.
Obviously, some low level of sloppiness is involved here, but at the same time, it makes sense that they would rush those things because the average player won't notice. Of course, that's not an excuse to make [word recalled] maps.

You would think they'd be much more careful with the elevators, since there's two in basically every map (aside from Old Aperture and BTS). One other amusing error/intentional thing in sp_a4_laser_platform is a rotating alarm light - it's actually a laser relay attached to the roof. (Random fact: the laserfield models in that map are named "only_deathfield_in_the_game".) There's also a misaligned texture in the model for the raised parts in the final boss fight - it's easy to tell since it shows the white part of squarebeams_rusty. Why does that texture have that section, anyway?

Valve's level designers can be incredibly lazy.

http://cubeupload.com/im/cS4kAk.jpg
Some Half-Life 1 tier mapping right there.