We've wondered about this very thing on the Classics list in the past, so from Notes and Queries, Number 191:

"_Quem Deus vult perdere._"--In Croker's _Johnson_, vol. v. p. 60., the
phrase, "Quem Deus vult perdere, prius dementat," is stated to be from a
Greek _iambic_ of Euripides:

"[Greek: Hon theos thelei apolesai prot' apophrenai]."

This statement is made first by Mr. John Pitts, late Rector of Great
Brickhill, Bucks[1], to Mr. Richard How of Aspley, Beds, and is taken for
granted successively by Boswell, Malone, and Croker. But no such Greek is,
in fact, to be found in Euripides; the words conveying a like sentiment

"[Greek: Hotan de Daimon andri porsunei kaka],
[Greek: Ton noun eblapse proton]."

The cause of this classical blunder of so many eminent annotators is, that
these words are not to be found in the usual college and school editions of
Euripides. The edition from which the above correct extract is made is in
ten volumes, published at Padua in 1743-53, with an Italian translation in
verse by P. Carmeli, and is to be found in vol. x. p. 268. as the 436-7th
verses of the _Tragedie incerte_, the meaning of which he thus gives in
prose "Quando vogliono gli Dei far perire alcuno, gli toglie la mente."



P.S.--In Croker's _Johnson_, vol. iv. p. 170., the phrase "_Omnia_ mea
mecum porto" is incorrectly quoted from _Val. Max._ vii. 2., instead of
"_Bona_ mea mecum porto."

[Footnote 1: This gentleman is wrong in saying _demento_ is of no
authority, as it is found in Lactantius. (See Facciolati.)

... I think this will get put in my Quellenforschung file later one ...