a) we need a LOT more context to understand what might have been happening at the time of this error ... can you please provide us with your configs, and more then just two lines of log output – we need to try to get a grasp on what all was happening with your solr instance just before (and during) this error.
b) we need to know exactly what code you were running – ie: you are clearly running code from trunk, but when from trunk? what svn rev? ... based on the path in the error, it seems like perhaps you are using http://solr-ra.tgels.com/ ... so perhaps the bug is there?
(it's very likely that this is in fact some core bug in Lucene/Solr, but we need to understand what tickled the bug ... it's trivial to see that a simple faceting on a field doesn't cause this problem in most situations – so we need to understand what was happening in your system at this moment that caused this problem)