When editing the tags of a question, a pop-up appears listing up to 6 tags containing the current word and their excerpt. When the excerpts are long, the tag box is truncated, and I can't scroll down to the bottom.
Here's a screenshot from https://math.stackexchange.com/posts/1141180/edit in Chrome 40.0.2214.111. I observe the same effect (with the text truncated about one line further up) in Firefox 35.0.1. The main bug is that there is a second line of tags (listing pde, differential-topology and differential-forms), but scrolling doesn't reveal it; pressing Down four times (to move the focus to the 4th tag, i.e. the first tag on the second line) makes the popup fully visible as it should be. A secondary bug is that the “How to Tag” box at the bottom right overlaps the footer and is hard to read because it lacks a background.
I haven't been able to reproduce the main bug on other sites. For example, on Physics, when the tag popup doesn't fit, a scroll bar appears and scrolling down makes the tag popup come into view. This makes me suspect that this is a specific bug with Math.SE's design, although it could be a combination of MathJax and long tag wiki excerpts.
I observe the same bugs here on meta while posting this question — see how the bottom of the excerpt of the [tag:meta-tag:bug] tag is truncated below.