The surest method of being deceived is to believe that one is cleverer than others.

[Le vrai moyen d’être trompé, c’est de se croire plus fin que les autres]

François VI, duc de La Rochefoucauld (1613-1680) French epigrammatist, memoirist, noble
Réflexions ou sentences et maximes morales [Reflections; or Sentences and Moral Maxims], ¶127 (1665-1678) [tr. FitzGibbon (1957), ¶127]
    (Source)

Present in the 1st (1665) edition. Another 1665 variant:
On est fort sujet à être trompé quand on croit être plus fin que les autres.
 
[We are very liable to be deceived when we believe ourselves to be more subtle than others.]

(Source (French)). Alternate translations:

The sure way to be cheated is, to fancy ourselves more cunning than others.
[pub. Donaldson (1783), ¶81; ed. Lepoittevin-Lacroix (1797), ¶123; ed. Carvill (1835), ¶69]

The true method of being deceived is to think oneself more cunning than others.
[ed. Gowens (1851), ¶130]

The true way to be deceived is to think oneself more knowing than others.
[tr. Bund/Friswell (1871), ¶127]

The surest way to be deceived is to think one's self cleverer than one's neighbor.
[tr. Heard (1917), ¶127]

The best way to be outwitted is to believe ourselves cleverer than others.
[tr. Stevens (1939), ¶127]

The surest way to be outwitted is to suppose yourself sharper than others.
[tr. Kronenberger (1959), ¶127]

The surest way to be taken in is to think oneself craftier than other people.
[tr. Tancock (1959), ¶127]

The best way to be deceived is to think ourselves more cunning than others.
[tr. Whichello (2016) ¶127]