Laudator Temporis Acti

Donna Zuckerberg, review of V. Liapis, A Commentary on the Rhesus Attributed to Euripides (Oxford: Oxford University Press, 2012), in Classical Review 63.1 (2013) 29-31 (at 30):

He shows how the play's vocabulary choices, particularly its use of extensive hapaces (pp. liii-lv), and its metrics (pp. lxiv-lvii) defy easy grouping with either Euripides' early or late works.

"Classical battle over Facebook founder's sister," Evening Standard (29 April 2013):

Professor Colin Leach, formerly at Oxford University, whose enthusiastic review is due to be published in the Journal of Classical Teachers later this year, expressed his surprise that Classical Review did not allocate so important a book to a senior academic. He also notes: "Zuckerberg used the 'word' hapaces, which purports to be the plural of hapax. But hapax is not a noun, but an indeclinable adverb, meaning 'once', and hapaces does not exist. It may be a feeble joke, or even an Americanism." God forbid.

One might wish that Zuckerberg's hapaces was itself a hapax, but no. See e.g. M. Gwyn Morgan, "The Long Way Round: Tacitus, Histories 1.27," Eranos 92 (1994) 93–101 (at 94, footnote omitted):

Then there is a pause (nec multo post), before his henchman Onomastus appears and delivers the message they have concerted as the indication that all is ready for their coup, a message requiring the use of two hapaces, one—if not both—of them a technical term (ab architecto et redemptoribus).

and Pär Sandin, Aeschylus' Supplices: Introduction and Commentary on vv. 1-523 (Lund: Symmachus, 2005), p. 171, n. 469:

To mention only a few hapaces (some recurring in late authors) on -µα, we find Th. 278 ποίφυγµα (on which see Sandin 2001), Th. 523 εἴκασµα, Ag. 396 πρόστριµµα, Ag. 1284 ὑπτίασµα (also Pr. 1005), Ag. 1416 νόµευµα, fr. 79 σκώπευµα.

The variant hapakes also occurs, regrettably, e.g. in Martin Bernal, Black Athena: The Afroasiatic Roots of Classical Civilization, Vol, III: The Linguistic Evidence (New Brunswick: Rutgers University Press, 2006), p. 124:

Szememrényi warns against accepting typological arguments on the ground that oddities or even hapakes (single exceptions) occur in all languages.

and Victoria Wohl, Euripides and the Politics of Form (Princeton: Princeton University Press, 2015), p. 155, n. 8:

Both polupēna and prosthēmata are hapakes that take humble words (the woof of a weaving, pēnē, or the prosaic prosthēkē, appendage) and turn them into unique coinages.

More common than either hapaces or hapakes is hapaxes. See e.g. Mark W. Edwards, The Iliad: A Commentary, Vol. V: Books 17-20 (Cambridge: Cambridge University Press, 1991), who uses hapaxes repeatedly on pp. 53-55.

Hapax is short for hapax legomenon, whose proper plural is hapax legomena. Some will defend the solecisms above on various grounds and call me a mossbacked linguistic prescriptivist, a charge to which I cheerfully plead guilty.