hudson.plugin.xunit.types.CustomType.CustomInputMetricDescriptor.getId() throws an UnsupportedOperationException. However, hudson.model.Hudson.getDescriptor(String) uses getId() to identify plugins since December 2010. The exception can be seen for example in the Hudson logs during jelly parsing.
(In my case, a more severe problem was losing the build history due to an internal plugin which uses Hudson.getDescriptor() - the exception prevented serializing the build.xml files to disk.)