Once upon a time the Romans broke their pledge when one of the Fabian family killed a Celtic tribal leader during negotiations, even though all envoys had been promised personal safety. The Celts demanded justice which was denied by the Roman Senate because the Fabians were powerful in Rome, and the Senators believed they were safe from retaliation because they didn't believe the crude disorganized Celts could muster enough force to demand satisfaction.

This second insult just angered the Celts more. The tribes united to march on Rome and demand revenge. They tore through the Roman countryside like locusts and laid siege to Rome itself for over half a year. This damaged the Roman economy so much that the Romans negotiated a peace for a blood price of one thousand pounds of gold. When the Romans had trouble finding that much gold easily, they tried to play for time when the gold was actually being weighed by claiming that the Celts were using faulty scales.

Now this was a really silly move, to add further insult to the injury already offered was only likely to anger the Celts even more. Brendan, aka Brennus, who was one of the Celtic war leaders, brought the petty carping of the Roman businessmen to an abrupt end by reminding the Romans they were getting off lightly. I imagine the assembled Celts must have really appreciated seeing Brendan throw his heavy Celtic sword onto the scales which increased the gold the Romans had to pay, while saying the famous line "vae victus" which literally means "woe to the defeated". The Romans saw sense, accepted the rebuke, and paid the extra gold quietly and quickly.