Here's a possible dilemma...
Let's say I work as a contractor modifying the Linux kernel (or any GPL source) for companies who require me to sign NDAs. Then I would give them the binaries and modified source per the GPL.
Then someone learns I've been modifying Linux and demands the modifications I did for every client. I must provide modified source code to 3rd parties per the GPL.
But NDAs prevent me admitting I ever made any modifications! Distributing modifications is an implicit disclosure (a gray area).