After quite some research without any results, somebody may help my forgetful brain:
I once have read an article or presentation about the problem, that in METAFONT the slanted K from Computer Modern has not neat tangents at the diagonal serifs. The reason for it is that lines are drawn by pens after the slanting and hence the pen is not affected by the slanting transformation (which would actually preserve tangents). There was even a picture for this (the serif path is only a outline, the diagonal is an outline with a drawn border). Quite probably, this article is freely available on the internet in pdf format.
Does anybody remember the author or the title of this article or presentation? Thank you very much for any hints!