I got caught with this by following my sat nav on the way to ride the Etape du tour in Briancon - grrr , The sat nav suggested a shorter route and I took it. I only saved about 3 miles.
I have occasionally been charged for class 2 on the autoroute, but I look out for this and when it happens I press the intercom and they change the tariff. Unfortunately I didn't notice on the Frejus tunnel till to late....