I'll give you an actual example:
The tiny village shop near to me was investigated, they were honest people who weren't fiddling anything but were naive. The tax people on the face of it could find nothing wrong, but they were very sharp.
They asked the shopkeeper, an elderly lady past retirement age what happened to the out of date pies. Oh we eat those ourselves she carelessly said. Ah, said the taxman , an undeclared benefit in kind, this is where it got really serious for them, HMRC made an assessment of £ 32,000 outstanding tax on the strength of it, which in due course was haggled down to £ 4,000.
This came directly from the lady's son who I went to school with. Don't get careless with HMRC it can be very, very costly, no matter how trivial you might believe the sums involved are!