As was already asked about in this phys.SE question many years ago—which, sadly, got closed and never received an answer—is there a collection of counterexamples in quantum information theory, "in the spirit of books like [...] Counterexamples in analysis "? Alternatively: assuming I have a statement I want to prove and Google didn't help me, is there a comprehensive list or similar I can check to see whether there already exists a counterexample to it? The fantastic post of Norbert Schuch on ca