I have observed twice that if
- I am using the mobile view (with Chrome for Android v18.0.something),
- my session has expired, but the page was loaded before it did (the time gap typically because I open-in-new tab several things to read, then don't read them all at once)
- and I click on a vote arrow,
then I will get an error popup with no error text, only "click to dismiss".
(If the page was loaded knowing I am not logged in (second point not holding), then I get a sensible "please login or register" popup, with an "×" button instead of "click to dismiss".)