Massive Increase in Billing

I have been a loyal customer of Vodacom for many years now, having on average 4 contracts going at any time with any incident. In December I was unpleasantly surprised with a bill of R5150.30 from my smallest R75 contract. This contract is a 1GB a month and has been going on for 2+ years now with no problems. It has always had a cap to stop unnecessary expenditure and now that the contract has been renewed, it appears that Vodacom has conveniently 'forgotten' to keep the cap on the contract. This is a terrible oversight on Vodacom's part and makes me feel that we as customers, cannot trust their services. My brother says he has had the same issue with Vodacom trying their luck and had the bill revoked. I will be taking this up with Vodacom, I am NOT paying this ridiculous bill.