I saw it once before when I was playing as the Arverni, and the only Roman fullstack, in the old days when the Camillan triarii were still capable of phalanx mode, attacked me on the ford outside Roma with my army consisting of Gaeroas, Bataroas, Botroas, and no gaesatae whatsoever. I was pretty much resigned to just taking as many of them as I could before I went, and surrendering the northern half of Italy after this defeat I was going to suffer, when suddenly they just did the same thing, line up in a column and just stand there on their side of the river.

I'll admit, I was so afraid of them that I actually waited for fifteen minutes on triple game speed to see if they would make a move. When they didn't, I sent my entire army en masse across the river, charged the leading maniples, which were the triarii, front, left and right with everything I had in one mad rush. I managed to break their six double-bronze triarii before the rest got the idea and started deploying, only to be caught right left and centre by my masses of barbarians who got among them and shattered them. It was a victory won through a bug, but hey, I wasn't arguing because it was better than I could have hoped for.

And then I got pissed and gave up the Arverni campaign because the recruitment zones weren't up yet and the Italian peninsula was incapable of recruiting anything south of Segesta, and I couldn't afford too many mercenaries.