Erasmus, Adagia, 1.1.20:
Ξυρὸς εἰς ἀκόνην, that is, the razor to the whetstone. This is usually said of those who have fallen by chance into those states of affairs which they did not desire. A razor is not able to fall more unfavorably than if it comes against the whetstone. The old saying of Horace is not dissimilar to this: ‘one seeking to slide his teeth against the coin ends up breaking them.’ This would be readily suited to one who, desirous of doing someone else harm finds someone by whom he is harmed in turn, and whom he cannot harm himself. Since indeed, a razor, if it run up against anything soft, cuts it apart; but if it runs against the whetstone, it is beaten back. Tarquinius considered this, when he said that he had in his mind that Actius Navius, the augur, would cut a whetstone with a razor, intending that the razor would have no power against the whetstone. Yet, the augur did what Tarquinius thought was impossible. Livy relates this in his first book.
Novacula in cotem (XX)
Ξυρὸς εἰς ἀκόνην, id est Novacula in cotem. Dici solitum in hos, qui forte in eas res inciderunt, in quas minime volebant. Neque novacula potest incommodius cadere, quam si in cotem incurrat. Ab hoc non ita multum abhorret illud Horatianum : Et fragili quaerens illidere dentem / infringet solido. Recte accommodabitur et in eum, qui laedendi cupidus tandem hominem nactus est a quo vicissim laedatur, cum illi nocere non possit. Siquidem novacula, si in molle quippiam inciderit, dissecat ; si in cotem, retunditur. Huc respexit Tarquinus, qui dixit sibi in animo esse, ut Actius Navius augur novacula cotem discinderet, significans in cotem nihil posse novaculam, quanquam ab augure factum quod ille fieri posse non credebat. Refert Livius libro primo.