In a textbox such as the one I'm typing this in,
doing C-k correctly kills the end of the line, but
C-y yanks back some other text (it's not the X selection,
so I really don't know where it comes from).
(this is with netscape-4.6, by the way)
C-y pulls in the netscape buffer, which may or may not be the same
as the X buffer, depending on what you've copied to it.