Millions of UK people may have paid the wrong amount of tax and it will either be clawed back or they will get a nice xmas present.
This is because HMRC used to use humans to check employers did their tax sums properly and changed to using computer checks and wrong sums have been missed. This has now been spotted somehow.
That is my summary of it based on the online stories anyway.
I do wish they would not go live with scare stories like this without also providing details of where the errors are so we humans can check our own employers sums.
I just hope that having a simple life in terms of no company car or childcare costs that I won't be affected.
p.s. according to another story it is the other way round. The computers have found errors that civil servants had been making for years. Now that sounds more likely doesn't it.
Well I reckon the computers have come so far that they are inventing errors that humans supposedly made in order to safeguard their own 'employment' 
People really should learn to understand and keep an eye on their tax codes. Inland Revenue has got mine wrong several times over the last couple of years and I've had to call them to correct it.
After my dad died leaving Mumsy with a limited widow's pension due to his long-standing illness/poor service record, she got a job and after a couple of years she had a letter like this - informing her the Tax Office had calculated her tax wrong and she had what seemed to us a huge amount to pay back 
She had to get a second job to pay it back and got terribly worried over the whole thing.
It was probably humans working it out in those days. Plus ca change...