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I've been mucking round in Godot lately trying to get some nice npr/toon shading stuff going and I got upset with reflections because they look smooth and realistic and boring. 

I don't want smooth and realistic and boring, I want my reflections to look like they're toon shaded as well. 

so..

To get toon lighting you posterize the result of your lighting calculations. The obvious answer to this would be to do the same thing to your reflections.   
sadly though, Godot does not let you get at the reflection calculations (neither do most engines afaik) so you must build your own. 

Sampling a cubemap is simple enough so no problem there but then you're kinda knackered. 
Making a nice toon style cubemap or posterizing a normal looking one kinda work but cubemaps exhibit a wide range of nasty artefacting  (filtering/texel density etc.)  and I don't like nasty artefacting.
Throwing resolution at the problem kinda works but not really 

so - I became sad and angry

But then I realised that if I posterized the vector used to sample the cubemap I'd get what I want. 
This isn't a 1 size fits all solution, but it works very well for what I'm trying to achieve.  

it's really simple too - just calculate the vector as normal then floor( (v * levels) / levels) before sampling the cubemap texture

I was just reminded of very early versions of corel painter (around v4) in which you could attach basically any tool setting to any input - eg. stylus tilt could control color and size while pressure controlled opacity and rotation (or whatever), all configurable on the fly etc.
it was a very long time ago so rose tinted glasses apply but my heart has yearned for that level of brush control ever since
(possible hyperbole)