The worst example of a scanner-hating sketch that
I've come across is small little Love Hina sketch
that I picked up a while back... the lines are on
one side, and there's faint colored shading on the
other.  The only way that I could even get it to
partially show up was to scan each side
separately, enchance the shading & then paste it
on top of a scan with the lines...  When looking
at the original sketch, I hadn't even realized
that there _was_ coloration until I held it up to
the light, so it's no wonder the scanner had
problem with it!