Box Office Bomb: The Last Knight was produced on a budget ranging from $217 to $260 million and not only received some of the worst critical reception of the series to date, but it also received staggeringly worse financial returns. It ended with a worldwide gross of about $600 million, $100 million less than the total of the first film, a movie released ten years prior and without 3D or nowadays' ticket prices; not to mention almost half of the gross of the last two films each (and it underperformed in the country that usually loves the franchise, China, grossing about $100 million less there than Age of Extinction). While $600 million is a healthy gross for most films, its an embarrassment for a film series coming off of two consecutive billion dollar movies that managed to make big bucks despite poor critical reception. Matter of fact, this was a very unhealthy gross for the film due to its colossal budget and analysts estimated Paramount lost a shocking total of $100 million due to the underperformance of The Last Knight. As a result, Paramount pulled all direct sequels to The Last Knight and after it was successful, Bumblebee, which was supposed to be a prequel film, was turned into a reboot film by Hasbro.
The Danza: Isabela Moner's character is named Izabella, while Glenn Morshower returns as General Morshower, who was named after him.
One of Hot Rod's lines in the scene where he drops Viviane off at Burton's castle ("She's very difficult!") is done by someone who is audibly not Omar Sy.
Baricade's last few lines in the film are also quite clearly not that of Jess Harnell.
Nitro Zeus is voiced by John Dimaggio in the main version of the movie, but by Steven Barr in the alternate version subtitled in non-English-speaking countries. The digital release, while mostly featuring John's performance, has a few lines from Barr's take of the character in a couple scenes.
Show Accuracy/Toy Accuracy: A particularly glaring one in the case of Barricade. Hasbro milked his Ensemble Dark Horse appeal a little by making the Deluxe figure with brass knuckles that had "Protect" and "Serve" engraved in reverse, meaning that he could leave those words on his victims like a brand. Then the movie comes out, and the brass knuckles not only say "Punish" and "Enslave" instead, but they're engraved normally.
Throw It In!: Screenwriter Matt Halloway is a huge Downton Abbey fan and joked that they should get Jim Carter (who plays Carson the Butler) to voice Edmund Burton's robot servant Cogman. Michael Bay decided to cast Carter after hearing this.
Uncredited Role: The voice actors for Canopy, Onslaught, Berserker, and the Guardian Knights are unlisted from the final product. As are the substitute voices for Hot Rod and Barricade.
In an interview with Empire Magazine, originally the idea of a Transformers film dealing with King Arthur and World War II were originally pitched as ideas for 2 separate movies, before it was decided to merge the 2 plotlines into one single film.
Tyrese Gibson was set to reprise his role of Robert Epps from the first three films, but his schedule prevented him from participating in the London-centric shoots. Strangely, despite Epps not appearing in the film, his absence was not confirmed until literally the day before the American release of the film.
The film was reportedly originally going to delve into the backstory of Optimus and Megatron, and there are hints of this in the final cut (in particular Megatron's "We were brothers once" line), but anything explicitly relating to this was apparently cut.
Supposedly, the writers were going to use the Quintessan race, as the previous film showed an organic hand and the creators are referred in plural. Hasbro pushed the idea of the "Original Prime" look, which creates some plotholes with both AoE and ROTF.
The toyline's renditions of Nitro Zeus and Cogman suggest that Cogman was originally going to have a larger combative role in the finale by connecting to Nitro's body after the Decepticon has his head blown off.