At the end of the concert when Morse has his photo taken, he's looking over his right shoulder. When the photo is printed in the newspaper, he's now looking over his left shoulder. The photo isn't a mirror image as Morse's parting is on the correct side. Therefore it is a different photo, with his left shoulder towards the photographer.
In a pub scene Strange gives Endeavour a newspaper and says "Late Edition." There is an article relating to the murder case. The paper is the Oxford Mail and the date is shown as Thursday July (the rest of the date can't be seen but it must be 1st July 1965.) Main story on page is Emerson beating Stolle in Wimbledon Men's Final. But this match was played the following day on Friday 2nd July 1965.
When Fred Thursday goes from the kitchen with a drink of stout and a glass for Morse they suddenly appear in the opposite hands as he enters the lounge.
A newspaper photographer took a photo of Morse in the concert lobby. However, the facial expression of Morse in the live photo is different from that in the photo that was published. In the live photo, his mouth is closed, but in the newspaper photo, it is slightly opened.
The orientation of the left foot of the dead woman in the train car changes from one cut to the other. In the first cut the toes are pointed horizontally. However, in the next cut, they are pointed vertically.
As Morse and his colleagues are looking at the sheet music of Rimsky-Korsakov's opera as part of their investigation, the name of the composer in Russian is misspelled on the cover page.
The title of the opera is misleading as well: it reads Snezhnaya Koroleva, which means Snow Queen in Russian. However, the real title of the opera is Snegurochka - Vesennyaya Skazka.
Snegurochka, correctly referred to as such by Morse, is a Russian fairy tale character that has nothing whatsoever in common with a Snow Queen (no such character exists in Russian folklore, by the way).
The opera's title is habitually translated into English as The Snow Maiden - A Spring Fairy Tale.
Endeavour identifies the piece of music associated with the entombment as the tenor's aria from Aida that is performed just before the tenor's entombment. The music is from Aida, but it is a duet, not an aria, and is performed after the tenor's (and soprano's) entombment.
Endeavour identifies a piece of sheet music as Ko-Ko's "list song" from "The Mikado". However, the sheet music is actually Ko-Ko's song "Tit-Willow".
At one point the doctor says, "They don't call it deadly nightshade for nothing," when referring to the plant drug digoxin. However digoxin (and digitalis) are derived from the foxglove plant (Digitalis purpurea), whereas the deadly nightshade plant mentioned by the doctor (Atropa belladonna) produces the poison atropine.
The record player that is set up to play the aria from Aida at Ben Nimmo's farm is open and remains stationary until the power is turned on enabling it to play. The atmosphere in the basement is clearly very dusty and the record should have accumulated a thin coating of dust in the time since it had been put in place (at least a few days previously), which would either severely degrade the sound it produced or made it unplayable. However the record is shown to be spotlessly clean and the quality of the sound is faultless.
Morse appears to have a copy of the 3LP set of Delibes' opera "Lakmé", with Joan Sutherland in the title role. This version was recorded in October 1967.
The railway yard where Evelyn Balfour's body is found features various items of rolling stock. Her body is in a dilapidated old wagon and we're told its ready to be scrapped The problem this scene is in the early 1960s and the wagon is a standard British Railways type built as recently as the 1950s, too soon for it to look so decrepit. Some of the coaches in the yard also suffer from the same problem, they look old and worn out but none of them can be more than 10 years old in the context of the story.
When Morse is examining the railway wagon using a torch (flashlight), he is pretty clearly using a Maglite. Maglites were not produced before 1979.