Depending on when you went to college, you may be thinking of the proton radius puzzle https://en.wikipedia.org/wiki/Proton_radius_puzzle https://en.wikipedia.o

Depending on when you went to college, you may be thinking of the proton radius puzzle https://en.wikipedia.org/wiki/Proton_radius_puzzle https://en.wikipedia.org/wiki/Proton_radius_puzzle which (to gloss over an enormous amount of detail) amounts to tension between two different ways of measuring the same property of the proton.