ArgumentException(message, paramName)
ArgumentNullException(paramName, message)

Ah, the many sucky APIs that will now stick with us because of this common misconception:

"It doesn't matter if we have different teams developing different parts of the same library, that's what documentation is for!"

@AlmondSauce In this particular instance ArgumentNullException inherits from ArgumentException, so there's little room for excuses!

@joycestick There's never room for excuses, but it still wouldn't surprise me if those classes were developed by completely separate people. The popular mantra back then was that anyone could develop any separate bit of code without an issue while just exposing the public interfaces, and there's lots of examples of APIs written in that era where similar things occur (I think the Java Swing framework is probably the worst example I can think of.)