I once heard that there was at least one case in which various Acharonim offered complex explanations to explain how the Rambam did not contradict certain Gemaras, and subsequently there were responsa of the Rambam found in which he was asked the very question they were answering, and responded that indeed that Gemara had slipped his mind when he wrote that.
Does anyone know of such a teshuva in the Rambam where he says that he made a mistake? I'm not as interested in matching the case of a mistake to the case of where commentaries offered explanations, but that would be extra brownie points.