When when I'm looking at my user page and click on the "visited 26 days, 19 consecutive" link, no calendar pops up. The calendar seems to appear for an instant (sometimes) but then disappears without really affecting the page formatting (i.e., no big gap where a calendar should be appears). This just started happening today (I was active yesterday and there was no problem), and happens for all my SE accounts (SO, Meta, Programmers).
The jQuery UI stylesheet changed, making the main
.ui-datepicker be hidden - a fix will be pushed in the next build.