I work for an advice agency and can assure you the tax credits system is a joke. Will they bugger your claim up or will they not? The trouble is - the calculation and rules relating to claiming are massively complicated and errors are often made inputting work details. One of the most common errors made was caused by officials at HMRC inputting working hours at (say 38 hpw) then forgetting to input the annual estimated income and stating earnings as NIL therefore claimants would receive the maximum tax credit. At the end of the year when you submitted your income details they would then realise the mistake and bill you for thousands.
Try arguing with the Revenue that it was there mistake and they simply say you should have checked your award notice more carefully, however these are also mega confusing to the average Joe. This is still happening. If you feel they have made a mistake complete form TC846 and contact your MP or nearest CAB or Law Centre.