I'm on Google Chrome 22.214.171.124 and was typing in a tag on a question on StackOverflow. The specific tag I was typing in was "language-agnostic". Around "language-ag" I tried clicking on the drop down to automatically insert the rest when nothing happened. After that the drop down stayed and never really went away. I could click and it vanished, but "language-agnostic" still showed if I clicked back in the tag input box. I tried reproducing it and was able to: it happens if the tag input box has an extra space in it. The simplest way to reproduce is to hit space in the box before even typing anything, and then trying to click the drop down when you type and suggestions show up.
Uncaught TypeError: Cannot read property 'length' of undefined. sstatic.net/so/Js/tageditor.js?v=6266:1