Claimants are assessed at the start of the tax year (or the start of their claim, whichever is later) and, unless they notify HMRC of changes to their income (or their circumstances) are paid the appropriate rate (based on the information they gave) for the rest of the year. Only at the end of the year, as you say, are their actual earnings examined and this is where claims for over payment originate. As a result, over or under payments are inevitable where a claimant�s earnings changes during the year (as many people�s do).
I do not believe claimants are under any obligation to notify changes of earnings mid-term and so would not face prosecution for failure to do so. However, I think you would be hard pressed to resist repaying amounts overpaid on the basis that the taxman knew of your earnings via the PAYE system and so should have adjusted your Tax Credits.
The Tax Credit system is a shambolic method of paying back money to people who should not have had it taken from them in the first place. It does nothing for the claimants (except makes them jump through hoops to get what is theirs and to suffer the worry of repayment when they are � almost inevitably � wrongly paid). We already have a perfectly adequate system to ensure people pay the correct amount of tax. The PAYE system used to be used to do this by adjusting individual�s tax codes to suit their circumstances. But that, of course, would not enable the Exchequer to be seen as quite so benevolent as taking money from people in the first place and then kindly giving it back.
It does, of course, keep an army of people employed at HMRC to sort out the mess (sorry - administer it) and it was the �flagship� of anti-poverty measures introduced by Gordon Brown as Chancellor.
So it must be alright.