I've done this twice.

I put up a notice board in my kitchen when I moved into a new place, and it looked squint even though my level said it was straight. I flipped the spirit level end-for-end, still straight - if the level was off and the workpiece was straight, then flipping it would make it read wrong.

Nope. Level was okay. Checked the wall, wall is plumb. Wait a sec - the wallpaper is not straight.

So the notice board went on lined up with the wallpaper, not reality.

The other one was fitting a six metre aluminium pole with a two metre aerial on top to a brand new multi-million pound building. The brackets and pole were absolutely straight and plumb. Got back down off the cherry picker, walked back across the yard to the van, pole looks really squint.

After much upping and downing and to-ing and fro-ing, it became clear that I needed to pack the aerial pole mounting to lean it over by a couple of degrees so it didn't look wrong!

The pole was straight, the multi-million pound brand new high end amazing building was distinctly on the piss with not one truly plumb vertical component anywhere.