The third "theorem" of Fermat, formulated in 1637, stood as a conjecture for a long time: although Fermat was claiming to have proved it, he did not share his proof and nobody was able to offer any for many centuries. Many "proofs" were discovered and later refuted, so the assertion kept oscillating between theorem and conjecture status, until Adrew Wile finally proved it in 1994 (having himself previously offered an unsuccessful proof). See https://en.wikipedia.org/wiki/Fermat%27s_Last_Theorem

The third "theorem" of Fermat, formulated in 1637, stood as a conjecture for a long time: although Fermat claimed to have proved it, he did not share his proof and nobody was able to offer any proof for many centuries. Many "proofs" were discovered and later refuted, so the assertion kept oscillating between theorem and conjecture status, until Andrew Wiles finally proved the conjecture in 1994 (having himself previously offered an unsuccessful proof). See https://en.wikipedia.org/wiki/Fermat%27s_Last_Theorem