Not at all. The XSD specification defines two operations called "strict validation" and "lax validation", and a processor may provide either or both. If it doesn't give you a choice then that's a limitation but not a bug. If it doesn't tell you which it's offering, then that's a documentation bug.
I agree that, there are "strict" and "lax" validation modes defined by XSD language. But as far as I know, "strict" and "lax" validation modes in XSD are related to wild-card validation (for e.g, <xs:any processContents="lax"/>). With the example that I had cited from .net documentation, the example said that, validate XML instance document x with XSD schema y, and as far as I could read from that example it was XSD "strict" validation, and that should have produced an XSD validation error or warning.
