[SirLuxuryYacht]

IMO, probably when the counterexamples outnumber the examples by which the heuristic is defined.

[coldtea]

And maybe even not then: the negatives from the counterexamples should also outweigh the positives from the heuristic.

If a heuristic has 100 majorly good applications, and 1000s of counter-examples of minor or no real consequence, it will still be good to use...