Selenium Server doesn't use the newest version of ant

I'm using a neat little tool called Selenium to automate some web tests (open browser, click here, etc), and running those tests in Fitnesse.  However, when trying to integrate my tests with some load-process-these-files work that another team member had done, I repeatedly got the error

Failed executing Ant target [run-job]: The type doesn't support the "osfamily" attribute.

Googling revealed that the Ant exec type didn't get added to Ant until 1.7, so you were likely using the wrong version of the ant.jar.  But I knew that both my Eclipse Ant plugin and my installed ant at ANT_HOME were both 1.7.1.  Mysterious.

Hours of troubleshooting ensued: turning all log levels up to TRACE (omg SO SO much output), poring over classpaths, deleting every errant ant.jar on the system (even those not in classpath), fresh checkouts.

My colleague finally added the -verbose option to the commandline when running tests, and lo and behold, we saw that ant dependencies were being loaded out of the selenium-server.jar, because it was first in the classpath.  Who knew those bad boys were hiding in there?

So the solution is to place the jar last in the classpath, so ant gets picked up from something more recent first.

Nearly four days later, I can actually start work on my tests again.  Whew.  Developers have straaaange problems.