We had a temporarily mysterious payment the other day.
The government had decided that we were deserving of tax credits again, something we haven't received since 2010. Fortunately a letter arrived a day or two after I noticed the payment go into our account and I felt sure that we did not deserve it. It was a "Working" tax credit and since neither of us has had any paid (self-)employment for 2 years, I telephoned them to clarify the situation.
It seems that with the furloughing business and Covid, quite a lot of people who had once received tax credits but had stopped, suddenly became eligible again. A week or two later the relevant Gov. Dept. wrote to me thanking me for informing them about the change in our circumstances and because it was only a small amount, they were not going to bother to get me to pay them back. So I'm about £26 better off than I ought to be.