During the chase scene at the end, it is shown on one monitor that the THX budget is 3,410 units over the budget of 14,000 units (24%). A voice had stated earlier that accounts are to be terminated when they exceed their original budget by 5%. When the account/chase is terminated, a voice is heard saying that the THX project is 6% over budget, which would be 840 units, not 3,410.
When THX has his first scene with SEN, an unseen woman asks SEN for his badge and he hands it to her. A minute later, as SEN follows THX, the badge is back on.
During the car chase, THX's car is shown from the front stopping in the right-hand lane of two. A subsequent shot showing a monitor has the car from the rear, stopped in the left-hand lane.
At the end of the car chase, the view from within the car shows it sliding into the scaffolding and the right side of the car hitting first. But the next shot from outside the car shows the car sliding into the scaffolding with the left side of it hitting the scaffolding first.
SRT's headphones as he gets into car.
David Ogden Stiers' last name is misspelled in the credits (as "Steers.")
When THX is shouldered, naked, by the police in prison, we can see his short shorts tan line on his leg - pretty impossible for people in an underground city. (Unless, of course, there are solariums/tanning salons in the city.)
During the car chase, the number 5 on a radio announcer's hat is backwards, revealing the usage of a mirror image.
The Hologram can eat.