I met very interesting problem. I install MikTeX on two PCs. In part of
texts that I work I use Babel package with Russian fonts and fancyhdr to
prepare header of page. To my surprise I discovered small difference on PCs.
On first I compile TeX without problems, but another returns me error that
it cannot find font for header of page and produces empty header. This is
important to understand reason, because I expect if I sent paper to somebody
else he can also compile this file.