I was repeatedly building, hitting ctrl-. to jump to the error and shift-f11 to rebuild, over and over again.

Suddenly ctrl-. didn't work and I got an assertion message. Clicking on the link with the mouse gave the same result. I
seem to remember filing this before, and Jesse adding some more logging to the assertion in case it should happen again.
Hopefully this trace is more helpful than last time:

java.lang.AssertionError:
this=Hyperlink[file:/Users/tor/netbeans/work/ruby/projects/src/org/netbeans/modules/ruby/rubyproject/templates/NewRubyFileWizardIterator.java:97:34:-1:-1]
ran=true encounteredThis=false modifiedThis=false
hyperlinks=[Hyperlink[file:/Users/tor/netbeans/work/nbbuild/templates/common.xml:93:-1:-1:-1]]
hyperlinks.contains(this)=true
	at org.apache.tools.ant.module.run.Hyperlink.updateLines(Hyperlink.java:154)
	at org.apache.tools.ant.module.run.Hyperlink.outputLineAction(Hyperlink.java:96)
	at org.netbeans.core.output2.Controller.sendCaretToError(Controller.java:760)
	at org.netbeans.core.output2.Controller.actionPerformed(Controller.java:376)
	at org.netbeans.core.output2.Controller$ControllerAction.actionPerformed(Controller.java:1293)
        ...