When in Netbeans IDE, SDI mode unselected tab is clicked in Explorer and other window was selected, IDE starts to switch between two tabs in Explorer. When you clcik on other window switching stops. I investigated it and event FOCUS_GAINED is sent to wrong tab, it causes call of requestFocus() on that tab in our code and then event FOCUS_GAINED comes for other incorrect tab and so on. It gets into infinite cycle.

It happens on Linux 2.4.1 (RH 6.2 with Gnome WM), JDK-1.4.0-beta-b63. Latest Dev build of Netbeans IDE. (http://www.netbeans.org/downloads.html)

Steps to reproduce:
1.Start Netbeans IDE in SDI mode.
2.Open Options window (Tools->Options)
3.Click on unselected tab in Explorer. IDE starts to switch between previously selected tab and clicked tab. (It does not happen always, so if it does not happen just select again Options window and click again on unselected tab in Explorer. Repeat this till it will start switching tabs.)
4.To stop switching tabs just select other window.

On Linux it happens often and switching does not stop, on Win2000 and Solaris
it switches tabs for a while and then stops without user action.