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I have seen this issue several times when NetBeans is not in focus and I click somewhere on the NetBeans IDE window to bring it into focus. Occasionally, NetBeans crashes, without any warning, and dumps an hs_err log.
Please reopen with more information. messages.log (under var/log/ in your userdir - http://wiki.netbeans.org/FaqWhatIsUserdir) and the hs_err log file.