Because that is what is going to happen at 5pm tomorrow. 
Two years ago I was offered a big piece of freelance work out of the blue, and had to pay tax for the first time in years. (I sold my soul to voluntary work more than a decade ago.)
Last year I did one tiny bit of work that came in from elsewhere as a result of that, and I earned the princely sum of £500. In November I called HMRC to ask whether I needed to file a tax return for such a small amount. Bloke said that as my income was under £2,500 I didn't need to file a return. He didn't ask any questions and a letter arrived a while later to confirm that I was no longer a taxpayer, provided I wasn't a company director, self-employed, various things.
Two weeks ago I was reading a newspaper article that suggested I was classed as self-employed and, no matter how small my income was, I had to file a tax return. There was a link for a questionnaire on the HMRC website and that decided that I was self-employed on 3 different counts.
I called HMRC immediately (14th Jan) to say that I had been advised wrongly in November, I needed to file a tax return, and had just tried to do so online, but I couldn't because it was showing up as having been "filed" on the date I was taken out of the system.
I was told that I could be put back on to self-assessment, but "the lead time for that department is 3 weeks, and they might not deal with it before the deadline"!!
I have been checking every day, but I still can't file the return and there is only one day left before the deadline. Unless there is a miracle tomorrow, I will be fined £100 and have to appeal against that, even though I don't owe a single penny in tax.
I am sooooooooooo utterly, totally, brassed off. 