You could be in the situation like a guy I worked with.
His name was Gareth Jones, and he was earning about 15K, he ended up paying tax and NI on his 15K and was also paying tax for another Gareth Jones in Scotland who was earning about 25K............ due to a tax office co**-up over NI numbers.
It took the tax office a few months to sort it out, if he didn't have any savings to rely on he would of been in real trouble.