That doesn't suprise me at all.
Over the years I have calibrated various scanned maps for Oziexplorer, and this involves entering a number of coordinate sets. The program then displays a lat/long grid which (if you have done it right) aligns well with the grid printed on the map itself.
On almost every map I do, I get at least one point wrong.... Would I take more care if flying with it, and if there was no easy way of checking the calibration? Probably yes, but am I
actually less likely to make a mistake? No. When commercial pilots are entering lat/long values, the other pilot is supposed to be checking every digit. Yeah, right, this is 100% safe, too
In modern flying (VFR or IFR) there is virtually no need to enter coordinates. If somebody is doing that, they are doing something very weird.