Note: Don't start looking yet! Everything is fine on the meta section. ;)
The Area51 site logo in the top-left corner is pixelated.
I noticed it with Safari 9.1.1 and Firefox 45.0. However, I don't think it's browser-dependent, just a matter of a low quality PNG.
That's not a major issue and it doesn't seem to be noticeable with regular pixel density displays, but I believe it should be reported.