If you install the Parameterized Trigger plugin in an older version of Jenkins, you can get a 404 in the validation area for “Build after other projects are built”. This is because Jenkins.instance.getDescriptor('BuildTrigger') (used from the /job/downstream/descriptorByName/BuildTrigger/check?value=upstream URL) may return either a hudson.tasks.BuildTrigger.DescriptorImpl or a hudson.plugins.parameterizedtrigger.BuildTrigger.DescriptorImpl, apparently at random.

4d33b6a should have fixed this in core 1.480. However it would be better for this method to actively check for ambiguities so similar problems do not recur.