Earlier, he’d tried a cross-kick when he had men screaming for the pass, but it is the final error that will haunt him forever.
On a sliding scale his error was nowhere near as bad as O’Mahony’s, but it cost the team dear.
And it showed the folly of persisting with Sexton at all times when his deputies get no chance to learn on the job.
There is no better example for Farrell than the performance of the support cast in Cardiff. Tadhg Beirne was immense, Robbie Henshaw sensational, Josh van der Flier a tyro and Andrew Porter was outrageously physical.