Windows 2000 Professional with Service Pack 4 and most critical updates, mIRC 6.12, also running Winamp 5, Opera, Microsoft Word and BitTorrent downloading. mIRC is set to minimize to tray without a password, and animate the icon when there is activity. I tested the bug by minimizing mIRC window consecutively 15 times, but the icon always showed up properly in the system tray and disappeared when I double-clicked it to restore the window. Very stable here, couldn't reproduce the problem.
