With the sample below, the resulting PDF will crash MuPDF, Adobe Reader
will show an empty page (though in a longer document it shows a message
saying the file is broken and just few pages are rendered while the rest
is blank), Evince will open the file and show the text correctly but
will give the following errors in the terminal:

Error: ExtGState 'GSoverprint' is unknown
Error: ExtGState 'GSknockout' is unknown
....

Rebeated several time.

Sample:

\setupcolors[overprint=yes]
\starttext
Hello \TeX.
\stoptext

ConTeXt version 2009.08.13 and 2009.08.16

Regards,
 Khaled

-- 
 Khaled Hosny
 Arabic localiser and member of Arabeyes.org team
 Free font developer