I'm using a FogBugz On Demand system, and working in Chrome. I've discovered that whenever I click on the checkbox of one of the cases, the browser starts behaving as if the Shift key is being held down - i.e. when I move the mouse up or down, all cases between the checked case and the mouse also get checked. I also cannot uncheck the box; have to refresh the browser to get things to clear.
No, my Shift key is not stuck down. I tried the same in IE, and it works fine. Closed my Chrome browser (all windows!) and reopened; same behavior. I even rebooted, just to be sure. Looks like this behavior is only in Chrome.