R 1800 account with R50 limit ???

When I first received my Ipad I ran up a massive account and with a result I put a limit on the account to send notifications when the bundle was running. Every month has work phenomenally well from then on. I never incurred a single cent over the R50 limit I had set. I then get an invoice for OVER R1700 for data and calls ?! On calling the call center I am told... the limits are not guaranteed and in the T's and C's it is stipulated there. So there is nothing that can actually be done unless of course I want to make a payment arrangement. Honestly?! You can not tell me that when you have a limit of R 50 set that the system is so unreliable that it incurs costs of over R 1700?? I can totally understand R 300 in excess and maybe R 600 at a push. Vodacom is very quick to just take your money before actually trying to sort out the issues first...that is if they sort it out. I have been a loyal customer since 1998..but getting a little bit annoyed at the lack of loyalty and customer service from Vodacom.