We're grandfathered in on our old Snap plan and recently went over our 200GB data cap. The account page says "your speed is now limited" or something to that effect, but in practice we were basically disconnected. It wasn't dial-up speed (I can still remember what using dial-up was like), no webpages would load at all. I had to go to my account page on my phone to get things running again.
I realise now I should've checked the FritzBox stats page for sync speeds at the time, but I didn't. Does anyone know what speeds you're supposed to be limited to after blowing your data cap? It seems like it's as close to zero as makes no difference.