A big problem is having to install the seal over the installed axle stub.
the seal therefore must be installed backward from the way seals are normally installed on shafts. This is also the case with installing the camshaft seals.
To avoid a potential leak, we carefully install the seal over the shaft, then take a feeler gauge and run it around between the seal lip and the shaft to be certain that the lip is not folded over on itself. If the feeler gauge does not go easily all around, the lip is most likely folded and the seal will not seal properly.
It is very easy to have a folded lip. This method insures the seal is installed correctly. We have had cases where some seals will seal even with a folded lip and some will not.