mscorwks.dll could not be loaded

i have sent a new application to one of my customers. suddenly they complain that they recieve a strange message that i have never encountered:
"mscorwks.dll could not be loaded". what the hell ? they have followed my instructions and have installed .Net Framework 1.1 and some additional libraries, so what might be the problem ?? good thing there's google and the .Net 247 community; and this article - http://www.dotnet247.com/247reference/msgs/41/207197.aspx.
so i have mixed a debug and release version ? hmm but why didnt i get that message locally then ? the Mystery of the Sith.....

Edit: so, after i read this thread i began to get suspicious and checked with my clients exactly what version of the .Net Framework they had and.... voila! they had 1.0.xxxx when 1.1 was needed! man, this remote work is getting so confusing sometimes....