... and forget to inform Tax Credits you've moved in.
Genuine mistake in my case.
I have been a lone parent and lived alone with my kids since 2006 (alone during pregnancy too).
March 2016 I moved in with my partner of 4 years.
I didn't inform Tax Credits until July 2016.
I now have a bill for £3,900 fortunately it can be repaid in monthly instalments.
My mistake was part stress related as we'd moved into partner's small one bed flat temporarily (4 people in there cramped and stressful) plus he works from home, and part disbelief that fines or overpayments were 'real' , and were just threats.
They're real.
My overpayment doesn't include a fine, but this is just to remind the odd person who like me might find that circumstance or in worst case scenario deliberate defraud intent, think they can get away without being fined or having to repay the overpayment.