Revealing mistake: In the beginning of the movie, when Hellboy is speaking on the phone with "Dad", he's speaking on on speaker, but when the phone shows up, instead of showing a screen of a call on speaker with the duration on the call, the screen shows what would appear if he was receiving a phone call before accepting or declining. (03:35:00)
Revealing mistake: A major selling point of the movie is the CGI technology used to de-age Will Smith. The effect works for the most part of the movie, including scenes in daylight, but in the last handful of minutes, maybe because of the plain 'happy-ending' light used, maybe for budget reasons, the face of Junior is thoroughly unconvincing, with a dramatic drop in quality in textures and modelization. (01:44:30)
Revealing mistake: After the Kree arrive on the ship, in a side shot one of them opens the lunchbox holding the tesseract. We see the blue light emanating from inside, but that only appears once the box is fully open - while it's opening, there's no light inside at all, despite the tesseract always glowing. A later shot shows the glow being visible around the edges of the lid even when the box is completely shut, and when Carol opens it later, the glow appears immediately.
Revealing mistake: The helmets the soldiers are wearing are made for airsoft/paintball players. No military would issue soldiers with helmets that had holes in them for field duty, as that could allow bullets or shrapnel to penetrate the soldiers' skulls.
Revealing mistake: The signal sweeps shown on the 'orca' device and on other screens throughout the movie are all identical.
Revealing mistake: In Guatemala, when Arnie shoots John, in the shots from behind Arnie's body double is noticeable.
Revealing mistake: After breaking Zapan's nose, Alita jumps on the table of two guys and trash talks them into a fight; before they attack, the two bottles of beer on the table are different, one being the generic red and white Cerveza, the other with a peach colored label. When she kicks the CGI bottles, those are two identical Cerveza ones. (01:01:15)
Revealing mistake: The fire Steve made when he "traveled in time" back to the Ice Age was obviously fake (computer-generated). The logs were arranged like a teepee and the "fire" was contained to the inner sides - flames did not spread toward the outer edges. Moreover, the fire was built on top of snow/ice but there was no melting under or around the fire.
Revealing mistake: When Nathan and Theresa are talking on the front porch, the background is fake (and mostly blurred). The white picket fence just doesn't look real. A hanging plant on the porch sways a little from wind but the two nearby trees (leaves/branches) are stationary. Next, one tree gets a gust of wind while the other one does not and the remaining trees in the background do not show any movement. (00:13:50)
Revealing mistake: The headlights on Hayley's car are fake (i.e, blocks of white to represent headbeams). (00:30:36)
Revealing mistake: When Anita goes to destroy their getaway car you see her pour gasoline all over the roof and hood. Then when she lights it on fire it's only burning inside the car. (01:06:40)
Revealing mistake: After successful surgery, Bruce's friend shows him the first page of a newspaper with the headline about the news, "The National Whisper." The 'date' is "November 2016", which is a bit odd (a monthly newspaper?), but even better is the fact that it just sports obvious gibberish all over. The title of the boxed article is "Geiare la goer uoa oge ou osgeou osd ogoernis pgoer aoubepre obieri ai beiragoer." (00:17:15)