Calculation of electromagnetic nucleon form factor in perturbative QCD is an old and seamingly over-discussed story, but there is still a shadow hanging over that straightforward calculation. Namely, there is factor of 2 discrepancy among results obtained by different groups.
The usual suspects were normalizations of the nucleon wave function. Different explanations were offered but without conclusive answer and the final statement was that
there are simply two different normalizations which both are equally reasonable. Consequently, the problem appeared: two different physical results. The unpleasant situation was circumvented by taking ratio between various amplitudes and everyone was happy. But no one should be. Years passed, we stumbled upon the problem, decided to reopen the case and definitely solve the mystery. The outcome was surprising and, of course, the answer was where no one expected it.