Crux (Latin for "cross", "gallow", or "t-shape") is a term applied by palaeographers, textual critics, bibliographers, and literary scholars to a point of significant corruption in a literary text. More serious than a simple slip of the pen or typographical error, a crux (probably deriving from Latin crux interpretum = "cross of the interpreters") is difficult or impossible to interpret and resolve. Cruxes occur in a wide range of pre-modern (ancient, medieval, and Renaissance) texts, printed and manuscript.
Crux (literary) Wikipedia
Though widely exposed to readers and scholars, the texts of William Shakespeare's plays yield some of the most famous literary cruxes. Some have been resolved fairly well. In Henry V, II.iii.16-7, the First Folio text has the Hostess describe Falstaff on his death-bed like this:
Lewis Theobald's editorial correction, "and 'a [he] babbl'd of green fields", has won almost universal acceptance from subsequent editors. Similarly, the "dram of eale" In Hamlet I,iv,36 can be sensibly interpreted as "dram of ev'l [evil]."
Other Shakespearean cruxes have not been so successfully resolved. In All's Well That Ends Well, IV.ii,38-9, Diana observes to Bertram,
Editors have reached no consensus on exactly what "ropes in such a scarre" can mean, or how it should be amended: "no satisfactory explanation or emendation has been offered." Perhaps the best alternative that has been proposed is "may rope 's [us] in such a snare." Another unresolved Shakespearean crux is the "runaway's eyes" in Romeo and Juliet, III,ii,6.
Sometimes a crux will not require emendation, but simply present a knotty problem of comprehension. In Henry IV, Part 1, IV,i,98-9, Sir Richard Vernon describes Prince Hal and his comrades as appearing:
This is most likely a reference to some obscure assertion about animal behavior, and has sent researchers poring through dusty volumes in search of an explanation.
In editions of Greek and Roman authors, a crux is marked off by obeli, to indicate that the editor is not confident enough either to follow the manuscript reading or to print a conjecture.