So there are 4 players left, 3 Town and 1 Cultist. The remaining town are two Jailors and a Mayor. At this point, the players are unsure of who the Cultist is, as the Jailors have not proven themselves yet. During the night, the Jailors both Jail the Mayor to prove each other's identity, but one of the Jailors get converted to being a Cultist. At this point in time, it is possible for the town to still win, as the Mayor can vote to lynch a Cultist and they won't be able to convert for this night, either.
Instead, the Jailor/Mayor wait to be converted by the Cultist and win with the Cultists.