This happened to me as well today. I got my final bill and there was a €50 exit fee added on even though I had been out of contract for a week before I switched. I rang their customer service and they removed it and said it had been applied 'in error'. There are bound to be people paying this fee that don't check their online bills and don't know that they have been charged it.
Hi Cait04, yes it seems to be pretty systematic doesn't it? Hopefully if people keep posting in here when it happens then if other people search online they will find this thread about it.