Eclipse 3.2 JUnit runner gets confused connecting to server?

I opened an Eclipse project today, ran a unit test, and got a socket exception I'd never seen before. The project was one I had set aside a few weeks ago after playing with the NetBeans 6 preview release.

After opening the project in Eclipse, I went straight to one of the JUnit test classes, made a small tweak to one of the test methods, then hit my usual Alt-Shift-X + T keyboard shortcut to run the test case with JUnit. Instead of seeing a green or red bar, Eclipse just sat there staring at me, saying it was running the test class with JUnit. The console view showed the red "terminate" button in bright red, indicating the run was proceeding, albeit at an exceedingly slow pace. After about 30 seconds, the console displayed:
Could not connect to:  : 3393
java.net.ConnectException: Connection refused: connect
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:333)
at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:195)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:182)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:366)
at java.net.Socket.connect(Socket.java:519)
at java.net.Socket.connect(Socket.java:469)
at java.net.Socket.(Socket.java:366)
at java.net.Socket.(Socket.java:179)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.connect(RemoteTestRunner.java:560)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:377)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)
A socket connection error? I was just trying to run a local JUnit test, not connect with any remote server.

My first theory was I must have been playing with remote debugging for this application a few weeks ago and configured Eclipse to connect with a remote JVM. I spent a minute going through the Eclipse configuration for the JUnit test to check out its settings. I saw nothing set for any remote JUnit connection. (I'm not even sure Eclipse's JUnit runner can do that.) Everything looked right, so I ran the test again and got the same connection refused exception.

My second theory was that I hadn't rebuilt the application since upgrading to JSE 1.6.0_01 from 1.6.0, and that Eclipse was doing its best to find a running 1.6.0 JVM to connect with. (This seemed far-fetched, but a rebuild only took a couple of seconds.) A rebuild didn't solve the problem.

My third theory was I had been using NetBeans for so long I must have forgotten how to run the JUnit test in Eclipse. Perhaps I was telling Eclipse to debug a remote application instead of running JUnit. I ran the test again, this time through the menu option. No luck.

That sent me searching the web for the solution. I found it pretty quickly, but not the underlying reason behind the problem.

The solution was to restart Eclipse. Why this worked I don't know, since I had just launched Eclipse minutes before. Apparently the JUnit runner thread in Eclipse attaches to an Eclipse server thread to run the tests. It would seem the client thread was trying to connect to the wrong port (3393) or that the server thread that had been listening on port 3393 for runtime requests failed. Either way, I would have expected Eclipse to log the error. Strangely, the only item in the Eclipse error log said:
Warnings while parsing the commands from the 'org.eclipse.ui.commands'
and 'org.eclipse.ui.actionDefinitions' extension points.
with a sub-message saying:
Commands should really have a category: plug-in='org.codehaus.groovy.eclipse',
id='org.codehaus.groovy.eclipse.debug.ui.testShortcut.debug',
categoryId='org.eclipse.debug.ui.category.debug'
Well, I did recently install the Groovy plugin. Did that cause the problem? If so, Eclipse thinks not being able to connect with the JUnit runtime is just a warning?

Anyone have the real answer as to what caused Eclipse to get so confused while trying to launch the JUnit runner? None of the web pages I viewed talking about the problem mentioned the cause for the failure.