I've recently started to notice a subtle rendering bug that I'm pretty sure didn't use to occur. It manifests rather eratically (Mac OS 10.6.8, Firefox 16.0.1); in fact I just realized that the display unpredictably changes between correct and incorrect on one and the same page without a reload, just from scrolling the spot out of and back into view.
The problem is that some inline math gets shifted upwards by one pixel and is no longer aligned with the baseline of the text. Here are screenshots from this question and this answer that show the correct and shifted rendering of two small pieces of inline math, a $1$, an $\inf$ and a $\min$:
[Update:] I found a particularly egregious example in a comment:
This is all one screenshot; note that the $B$s are OK in Haskell's comment but shifted upward in mine; the lowercase $b$s look particularly bad. (View the image on its own to get the full resolution.)