It's not your brain being dumb. This did indeed happen, or at least Tacitus says that it did (in Annals XIV).

The Roman general in charge of governing Britain at the time was Gaius Suetonius Paulinus. In 60/61 CE he and his troops ventured to northern Wales to invade the Island of Mona (Anglesey), which was a stronghold for celtic druids. When he and his troops arrived on the shores of the island they found there waiting a large group of men and women holding torches encircled by druids, lifting their hands to the skies and cursing the Romans. At first, Paulinus' men were shocked by this display, until he kicked some sense back into them and they charged. The islanders didn't defend themselves, but rather let themselves be killed willingly. This, in combination with the circular arrangement of the druids, their chants and the torches, gives off the impression that this was indeed a human sacrifice in which the Romans unknowingly participated.

To make matters worse, there was an uprising while Paulinus and his troops were busy dealing with the druids of Mona by the queen of the Iceni tribe, Boudica, of which you may have heard.

Edit: spelling