When the parser cannot work out the meaning, it tends to report "unexpected input". There is a ticket about parser recovery that may address some types of errors like the extra closing paren in your example. But it has not gotten off the ground.
https://issues.apache.org/jira/browse/GROOVY-9192 You can also keep an eye on all the Parrot Parser bugs here: https://issues.apache.org/jira/issues/?jql=project+%3D+GROOVY+AND+component+%3D+parser-antlr4
