The following error message often appears in my atlassian-jira.log :

2009-01-13 14:13:24,925 http-8080-2 ERROR [bc.issue.search.AbstractIssuePickerSearchProvider] Error while executing search request
java.lang.IllegalArgumentException: fieldName must not be null
(see the full stack trace in the attachment)

The error message comes from Lucene : http://svn.apache.org/viewvc/lucene/java/trunk/contrib/analyzers/src/java/org/apache/lucene/analysis/fr/FrenchAnalyzer.java?view=markup
A similar bug as been reported for Confluence : CONF-12711
Something similar has been reported in another application : http://www.nabble.com/lucene-FrenchAnalyser-bad-implementation-td15561023s13679.html
and their workaround was to delete the line throwing the exception : the "fieldName" variable is not used, and there's nothing similar in other languages.

I suppose the bug is in Lucene, but I'm not sure.