I read the following paragraph from Serre's book (Topics in Galois Theory).

Although the proof of the classification theorem has been announced, described, and advertised since 1980, it is not yet clear whether it is complete or not: the part on "quasi-thin" groups has never been published

I am confused as I have read that classification of finite simple groups is complete.

See the first four lines on *Wikipedia*.

`Theorem — Every finite simple group is isomorphic to one of the following groups: a member of one of three infinite classes of such, namely: the cyclic groups of prime order, the alternating groups of degree at least 5, the groups of Lie type one of 26 groups called the "sporadic groups" the Tits group (which is sometimes considered a 27th sporadic group).`

Even if I assume that Wikipedia is wrong on this matter, what does he mean by saying that we don't know whether it is complete or not.