This happens in any Flash application that hides or customizes the cursor, often games. Whenever I click outside the Flash area, even if it's just the scroll bar or the tabs bar, the default cursor starts flickering in the Flash area where it should be hidden. At first it seemed to be a Flash issue, but this doesn't happen in Chrome or IE.

I'm not sure if this is related but it's worth mentioning. In some cases, the cursor keeps flickering outside the Flash area, even getting to disappear sometimes, forcing me to close the tab or terminate plugin-container.exe.