Most Bizarre Operatic Plot Twist Ever?

From the liner notes to Christophe Rousset's recording of Tommaso Traetta's Antigona, first produced in St. Petersburg in 1772:

The outline of the opera closely follows Sophocles' tragedy, and respectfully observes the ethical background and those elements of "Pity and Terror" which Coltellini [the librettist] considered indispensable. The conclusion, however, is different: in place of the original catastrophe, ideological reasons dictated a happy ending which is then followed by a lavish divertissement in the French manner, in the purest tradition of theatrical entertainments. It is, in fact, an ending of great emotional power which should have satisfied even the severe Empress of Russia, since Creon's clemency does not appear as a weakness, but rather as an enlightened action. It is an act of conscience on the part of a tyrant who, although he is the stern custodian of the harsh laws dictated by reason of state, realises that even kings and their wishes are subject to the light of Reason, which succeeds in forging a link between absolute power on the one hand, and on the other, a dawning consciousness of human rights.

This is, of course, far from being the largest attested difference between an operatic plot and its classical source. It is a later paragraph in the CD booklet that inspired my title. Ten years before Antigona, "another splendid collaboration between Traetta and Coltellini" was the Ifigenia in Tauride:

The earlier opera, however, presented at the Palace of Schönbrunn, ends tragically, even cruelly, with the protagonist savagely killing the lover she has repeatedly scorned -- an ending that flies in the face not only of all operatic librettos, but also of Euripides himself.

Huh? Who is Iphigenia's lover? Thoas? Pylades? Some invented character unknown to Euripides?