Well I've heard this talk of Nvidia not following TSMC's guidelines on 40nm design from a few different sources now. If there's any truth to that it could be coming back to bite them in the ass so who know what they're scrambling to fix at this point.
The way this usual work is like this: the fab provides a deck with design rules. The fabless company does place and route and circuit design against those rules. Before tape-out, the fabless company does many DRC runs until the GDS2 file is clean against the design rule deck. When the fab receives the GDS2 file, it runs DRC on their site to, otherwise the tape out is rejected.
The one place where DRC violations are acceptable is for memories: the fab provides a basic 1-bit memory cell that can be used by a design house to build their own RAMs. Those cells violate the standard DRC deck, but are pre-approved by the fab.
All other DRC violations have to be explicitly signed off with the fab first, otherwise you're on your own.
Most companies won't bother because, other than RAM's, there's really not much to gain. It's possible that big boys do it for whatever reason, but it's not possible to sneak it past the fab.
Now that's specific for DRC only. There are many other parameters, of course. Circuit design is done against the approved spice deck of the fab. And much more fluid also. (The fab adjust parameters over time. Pretty much always making performance worse.) Noise calculations are ultimately also derived from that, but there are steps in between to simplify the Spice model into a still simplerer model etc.
Anyway: you heard multiple sources talking about Nvidia not following the rules. It's the first time I hear about this. (Actually, I've never heard any rumors about any company not following the rules.) Can you give some pointers?