The new National Insurance and PAYE Service (NPS) went live in a phased implementation that started in June 2009. This itself was two years' behind schedule. The reasons stated for the delay were to protect the integrity of the PAYE system. However, the delays resulted in a huge backlog of PAYE returns in 2007-08 and 08-09.
Despite the delays to ensure integrity the department discovered that going live with a high volume computer system that wasn't adequately scoped or tested was a sure way to foul things up even more. In 2009/10 PAYE processing errors increased amounting to £132m in underpayments (a 15.7% increase) and £238m in overpayments (a 148% increase).
For a more detailed review of the errors see the AccountingWeb website