VS/BTS Error "The system cannot find the file specified"

If you receive the error message "The system cannot find the file specified." with the exception code 80070002 while adding a disassembling schema in Visual Studio/BizTalk, it is because VS does not find the assembly from the referenced project with the schema. Just build the referenced project, and then you should be able to add the schema.

XML-Error Message "content is not allowed in prolog"

When we created a BizTalk Mapping in VS2008, the compiler returned the error "line 1 colum 1 content is not allowed in prolog". Since the compiler told us that the error was detected in the very beginning (line 1 column 1), we needed a bit of time to see that actually the error was inside the document, where we had filled an element with a value although this shouldn't contain one.
So, if you get the same error message, check which fields you filled and if one of these shouldn't be filled.