The second worst error(s) is quite similar. Over a limited flight time by the Aries to the Moon, the Moon changes phases out of proportion to the amount of time involved, in some cases going back in time. Leave it to an ALPO member to pick that up at the premier.
Plus the impossible visual angle of the crescent Earth/Moon over the monolith.
But compared to the usual space scifi dreck (except for Forbidden Planet and This Island Earth), it was such a leap technically and philosophically that these lapses were easily (if temporarily) forgotten.
BTW, I always thought it was a nice touch to have Aries (the ram) bring humans to the Moon.