So, here I am, in the middle of some intense debugging related to the dreaded combination of threads, class loaders and deadlocks and suddenly ... my integration tests no longer run!
Ultimately, my changes were very modest (a little bit of extra synchronization against the context class loader), so what gives?
Well, if your tests are like mine, and run against Firefox, your issue might be that Selenium is unable to start up Firefox if an upgrade has occurred, especially if the Firefox upgrade is not compatible with your plugins.
The solution? Start Firefox manually, to get through the dialogs it presents. Then rerun your tests.