1st.. you need to add ".c" to your filenames
SED isn´t able to put the .c by itself in.. yo..
"mandelbrot_pure" should be "mandelbrot_pure.c"
second.. this seems to be a Bug in SED, cause it happens to many people these days:
http://www.opserver.de/ubb7/ubbthreads.php?ubb=showflat&Number=390289#Post390289perhaps it´s a problem with a antivir program or the SED itself (tried to reinstall?)