In 2010 R. Pohl and collaborators using muonic hydrogen surprised the world by finding that the proton charge radius measured with muons is different from the proton radius measured with electrons . In the three years since, there have several confirming results that combined indicate the muon-measured proton radius is about 13 times more precise and 7.2 (larger electron) sigma smaller than the electron-measured proton radius.
Many proposed solutions to the puzzle are now known to be wrong, leaving at present the three least unlikely explanations as (1) novel beyond standard model physics, (2) novel two-photon physics, or (3) issues with the electron-proton experiments. Several new experiments are also in various stages of development. I will describe the puzzle, some possible and some wrong solutions, and the new experiments.
 R. Pohl et al, Nature 466, 213 (2010)