"...could it be an accidental diopter adjustment?"
I rather doubt it. If the diopter is maladjusted nothing would be clear. OP claims good focus through the viewfinder.
EB
EOS 1D, EOS 1D MK IIn, EOS 1D MK III, EOS 1Ds MK III, EOS 1D MK IV and EOS 1DX and many lenses.
