Sophie's glasses start out as reading glasses, but later she wears them all of the time and not just when reading.
When Fleshlumpeater steals BFG's fizzy water, BFG puts the vegetable on the table. But directly after Sophie climbs out of the vegetable the fizzy water bottle is seen back on the table for a second.
[French version only] The tags on the dream bottles are translated in French. However, after Sophie comes back to Giant Country, we see a close-up shot of the bottles with French dream titles and a couple of shots later, the titles seen in the background are in English.
Right at the start of the film when Sophie comes back into the dormitory at 3:00am, she leaves the door open. Then when she is in bed reading and hears someone (matron) coming, you can hear the door opening and then see it being thrown open as matron enters, as though it had been closed.
When the Queen is in residence, the Royal Standard is the flag that should be flying above Buckingham Palace. In this film, it is the Union Flag. This should only fly when the Queen isn't at home. It used to be the case that the Royal Standard should be the only flag above the palace and when the Queen was away, there would be nothing. However, there was an uproar following the death of Princess Diana as Queen Elizabeth II wasn't home so there was no flag to put at half mast. Since then, it's the Royal Standard when she's in, and the Union Flag when she's away.
After the grabbing Sophie, BFG hides by jumping and lying down on a truck. The tires and truck must be of superb construction because neither react to this sudden huge weight.
During the opening when Sophie is reading her book she uses a torch with an obvious LED bulb. The film is set in 1983, way before LED torches were a thing.
According to the postage date of one of the letters Sophie puts on the desktop, the events take place in September 1983. But when the Queen phones foreign presidents (suggested by the fact she tells to Nancy (Nancy Reagan) that she want to talk to Ronnie (Ronald Reagan)), she talks before with Boris. Assuming this refers to the Russian/Soviet leader, Boris Yeltsin was elected in 1991. Yuri Andropov headed the USSR at this time.
The BFG took Sophie away and back to Giant Country because, as he explained to her, she had seen him, so her couldn't let her back. Yet not much later he was putting her back in the orphanage even though she didn't want to go. So his initial explanation for taking her no longer made sense.
All of the Queen's equerries, assistants and servants address her as "ma'am" which rhymes with farm. However, it is well known that the real Queen Elizabeth II hates this and prefers being called "m'am" which rhymes with jam. As close aides to the Queen, they should really know this. Funnily enough the only person who gets it right is Sophie, who could be forgiven for getting it wrong.