JQL with thrown an exception you try to run a query where the function is disbaled.

java.lang.IllegalStateException: There is no OperandHandler registered to handle the operand 'blah()'.
	at com.atlassian.jira.jql.query.GenericClauseQueryFactory.getQuery(GenericClauseQueryFactory.java:78)
	at com.atlassian.jira.jql.query.ReporterClauseQueryFactory.getQuery(ReporterClauseQueryFactory.java:35)
	at com.atlassian.jira.jql.query.QueryVisitor.visit(QueryVisitor.java:135)
	at com.atlassian.jira.jql.query.QueryVisitor.visit(QueryVisitor.java:24)
	at com.atlassian.query.clause.TerminalClauseImpl.accept(TerminalClauseImpl.java:137)
	at com.atlassian.jira.jql.query.QueryVisitor.createQuery(QueryVisitor.java:65)
	at com.atlassian.jira.issue.search.providers.LuceneSearchProvider.getHits(LuceneSearchProvider.java:162)
	at com.atlassian.jira.issue.search.providers.LuceneSearchProvider.searchCount(LuceneSearchProvider.java:94)
	at com.atlassian.jira.portal.portlets.SavedFiltersPortlet.getCountsForFilter(SavedFiltersPortlet.java:53)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:267)
	at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:197)
	at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:175)
	at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:220)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:55)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:70)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:55)
	at org.apache.velocity.runtime.directive.Foreach.render(Foreach.java:166)
	at org.apache.velocity.runtime.parser.node.ASTDirective.render(ASTDirective.java:114)
	at org.apache.velocity.runtime.parser.node.ASTBlock.render(ASTBlock.java:55)
	at org.apache.velocity.runtime.parser.node.ASTIfStatement.render(ASTIfStatement.java:70)
	at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:230)
	at org.apache.velocity.Template.merge(Template.java:256)
	at org.apache.velocity.app.VelocityEngine.mergeTemplate(VelocityEngine.java:450)
	at com.atlassian.velocity.DefaultVelocityManager.getEncodedBody(DefaultVelocityManager.java:77)
	at com.atlassian.velocity.DefaultVelocityManager.getEncodedBody(DefaultVelocityManager.java:61)