I looked at out code a little more and noticed that I was ignoring
exceptions on rs.close and stmt.close. There was actually an exception
preceding the one that I mentioned in my previous email:

org.firebirdsql.gds.GDSException: request synchronization error
at org.firebirdsql.jgds.GDS_Impl.readStatusVector(GDS_Impl.java:1414)
at org.firebirdsql.jgds.GDS_Impl.receiveResponse(GDS_Impl.java:1351)
at org.firebirdsql.jgds.GDS_Impl.isc_dsql_free_statement(GDS_Impl.java:8
87)
at org.firebirdsql.jca.FBManagedConnection.closeStatement(FBManagedConne
ction.java:622)
at org.firebirdsql.jdbc.FBStatement.closeResultSet(FBStatement.java:853)
at org.firebirdsql.jdbc.FBResultSet$FBStatementFetcher.close(FBResultSet
.java:2519)
at org.firebirdsql.jdbc.FBResultSet.close(FBResultSet.java:178)

The exception is thrown because an isc_req_sync (335544364) error comes
back from the server. I haven't upgraded to firebird 1.0 from the rc.
Could this be a problem with the server?