Soon after the construction of the Monster in 1981, a proof, totaling more than 10,000 pages, was supplied in 1983 by Daniel Gorenstein, that claimed to successfully list all finite simple groups. This was premature, as gaps were later discovered in the classification of quasithin groups. The gaps were filled in 2004 by a 1300 page classification of quasithin groups and the proof is now generally accepted as complete.
Soon after the construction of the Monster in 1981, a proof totaling more than 10,000 pages, was supplied in 1983 by Daniel Gorenstein that claimed to successfully list all finite simple groups. This was premature, as gaps were later discovered in the classification of quasithin groups The gaps were filled in 2004 by a 1300 page classification of quasithin groups and the proof is now generally accepted as complete.