I doubt it's designed to silently break in some cases. Unrealistic isn't realistic until one day it is and that is a bad day. I suppose it could just throw an error in the case of a hash collision, but I doubt it.
But what does it do, then? The page I links states that it uses a hash table. Hash tables apply a hash function to the key. Hash functions map arbitrary input data onto data of a fixed size. It's inevitable that collisions will occur. ~~even if you use some sort of clever workaround in the case of collisions, eventually you use up all the available outputs.~~ (my bad)
I'm not claiming that it will silently break! I'd be very interested in exploring the internals a little more and finding out how hard it is to get a collision in various implementations and how they behave subsequently.
EDIT: I've read chasil's comment and agree that it must be storing raw keys in the array. I guess awk uses separate chaining or something to get around hash collisions.
