The Imaam forgot to recite the fourth Takbeer of Janaazah Salaat.

Answer

Four Takbeers in Janaazah Salaat are Fardh. If the fourth Takbeer was not recited, then the Janaazah Salaat was not valid. They were supposed to have repeated the Salaat. But now it is too late. However, if some people in the group had recited the fourth Takbeer, then the Salaat was valid.