In atoms, electrons normally orbit the nucleus of an atom. The nucleus can contain a mixture of protons and neutrons in most atoms although hydrogen just has a single proton, which makes it the simplest atom to study: there’s just one electron orbiting one proton.
One day a bunch of scientists wondered what would happen if they evicted the electron from a hydrogen atom and instead replaced it with a muon. A muon is part of the same family of particles as the electron — a family collectively called leptons — and it has the same charge and spin, but it’s 207 times heavier and doesn't exist for very long.
What they noticed was that, against all known physics, the proton seemed to shrink by 4% in the presence of the muon. This elicited much scratching of heads and considerable stroking of beards. In fact, hundreds of papers were written about it suggesting the new laws of physics that might have been discovered.
Alas, it all came down to a faulty ruler.
They thought the standard proton (with an electron orbiting it) was 0.876 femtometers and measured the muonic proton to be 0.84 femtometers.
But some clever-dick has come along and measured the standard proton with a better ruler and pegs it at 0.833 femtometers +/- 0.01, which removes the discrepancy found with the muonic proton.
I feel sorry for all those scientists who expended much brain-power coming up with new theories, although I did giggle a bit.