Plot hole: When Cyborg tells his origin story, he says that the Mother Box in possession of the government was shelved till after the night when Superman died, when it came to life leading it to be studied by Cyborg's father, Silas. However, that contradicts the previous movie, Batman v Superman [both 1 h 39' and 2 h mark], where Batman came upon Luther's metahuman evidence file, including Silas Stone's vlog with the creation of Cyborg that Diana watches. (01:02:35)
Continuity mistake: When Dr. Silas Stone is speaking with Victor, he is wearing a completely different set of clothes from what he was wearing at the lab in the scene just prior. It is implied that the scene happened immediately when coming home from the lab.
Factual error: The existing Justice League members realise that they cannot battle Steppenwolf without Superman, so they procure the last Motherbox to resurrect Superman from death. Unfortunately, the crippled Kryptonian spacecraft lacks sufficient power to activate the Motherbox. The Flash suggests that, given enough distance to accelerate, he can use his super speed to generate an enormous static electrical charge to activate the Motherbox. The problem with this scenario is that, although the Flash may generate a huge static electrical field at super speed, he is constantly discharging that static electricity, as we see every single time he exerts his power. As Flash races toward the Motherbox, gigantic arcs of electricity (easily hundreds of thousands of volts) pour off him, grounding to the spacecraft's bulkheads, thus neutralizing the static charge. Meaning that The Flash is not accumulating energy, he is discharging energy with every step; so, by the time he arrives at the Motherbox, he should have no more accumulated static electrical energy than if he started ten feet away from it.
Character mistake: At the start Wonder Woman stops a terrorist attack in London, and one of the terrorists tells her the bomb will flatten 4 blocks. This must be true as she is using her lasso of truth. But she just throws the bomb through the roof window and it explodes without damaging anything. A bomb with that blast radius would still damage nearby buildings, whether it detonated in the air or on the ground.
Continuity mistake: When Wonder Woman says "He's back" looking at the freshly resurrected shirtless Superman, the policeman by the patrol car is leaning against it differently compared to the previous line-up shot; he was lowering the hand with the radio, and he's on the contrary talking on it now. (01:13:35)
Continuity mistake: Aquaman puts Clark's body in the amniotic pool. The photo of his father falls in the water-like liquid and begins to sink, and quickly. Minutes pass with the hacking and all the discussion in between on how to power the process, but when Flash finally 'jumpstarts' the machine, a shot is dedicated to the photo that is still there in the pool, yet to be submerged. (01:10:35)
Continuity mistake: When Aquaman dumps the sailor that was attacked by parademons, the man has no green stains on himself, but he exhibits large ones in a close-up, not to mention that Aquaman's hand is clean in the rest of the scene other than the glass close-up (as we can see from the Snyder cut, the encounter had nothing to do with parademons in the original concept). (00:42:40)
Other mistake: The janitor, Howard, stops Dr. Stone to talk to him about the loss of his son. Look at his badge; the person on it sports a beard and sideburns and a hat, and it's "Jensen" plus initial. Later on the same janitor hears a noise inside the lab and you can see the badge picture is different and it lists the full first and last name. The first scene was a Whedon reshoot, so probably after weeks they didn't have the original prop and made another one. (00:18:55 - 00:21:15)