When I hover over steam or other tags in the list of my favourite tags, the mouseover popup is shown partially off-screen:
The page fit on-screen before the pop-up, and afterwards a horizontal scrollbar shows up. I can use my arrow keys to move over (and attempt to keep my mouse above the tag), but that's not ideal behavior.
I use Chrome 11.0.696.57 beta; also occurs in IE 8 and Firefox 4.0 and 4.0.1. All on Windows XP.
Now it's just the Related Tags, slightly lower on the same sidebar, that exhibit this behavior. (Favorite and Ignored work properly, as Jin describes in his answer.)
Edit 2: Still happening for related tags.