Factual error: En route westwards to Cairo, Lawrence is pictured arriving supposedly at the east bank of the Suez Canal but the direction of sunlight from the right with the Canal in the background can only mean that Lawrence has already crossed the Canal and is standing on the west bank.
Factual error: On his way to Cairo, history records that after crossing the Sinai Desert, Lawrence arrived at the sea at Akaba (which is at the northern end of the Red Sea). In the film he turns left along the beach when he should have turned right (towards the Suez Canal). The scene was shot on a Mediterranean beach in Spain (from where Cairo WOULD be on his left!)
Continuity mistake: Lawrence & his Arab revolt are supposedly travelling up the right flank of the Army (briefing scene with blackboard) where Harry is told to get with the arab cavalry, but it's the guns that matter, which are supposed to pound the turkish centre. In the later scene where the Arab revolt is pursuing their journey up the right flank into the night, they see gun flashes in the distance and Lawrence says god help them.... he is looking over his right shoulder as he is riding, to look at the barrage. It should be his left shoulder as they are travelling with the turkish centre to their left.
Other mistake: It's settled that Lawrence takes 50 of Prince Faisal's men with him to fight the Turks in Akaba (as long as they get there through the burning desert). But to my calculations there's only 32 men riding with him, plus those young servant wannabe's who (without permission) follow the party.
Revealing mistake: Near the beginning, after the corporal has burned his finger and tells the other corporal that Lawrence is balmy, O'Toole begins to run up the stairs to meet with a general. After O'Toole is out of view of the camera, you can tell he has stopped running up the stairs by his shadow on the wall not moving, because he knows that the director is about to shout "cut".
Factual error: There are a number of mistakes with the Vickers guns used by the Arab forces in the film. Quite a few of them are smooth jacketed guns, not introduced until October 1918. Secondly, the one on Colonel Brighton's armoured car not only has the smooth jacket but has a blank fire adaptor very prominently at the front.
Character mistake: On the journey to meet Prince Feisal, Lawrence is offered food by his guide and accepts using his left hand. This would have been considered an insult as Arabs always eat using the right hand. The historical Lawrence would have known better. He had spent many years in the Middle East before the war.