In the preceding episode ('The Empty Child') the Doctor states that "mauve is the universally recognised colour for danger" and that red (the colour human beings most associate with alarm or danger) is, by everyone else's standards, "camp". However, when the Doctor attempts to open/enter the Chula ambulance ship in 'The Doctor Dances', the alarm that is triggered shows a blinking red light.
As the gang start running from the child, who is breaking down the wall to get at them, there is a short sequence in which you can see the hole they dived through, the one Jack supposedly "digitally rewound".
As the child arrives in "his" room, we're shown a full reel spinning, which supposedly accounts for the noise of the ended tape. However, the reel keeps changing spinning speeds in the next few shots, in some of which the edge of the tape strikes the recorder decidedly off beat.
A patient-doctor conversation has been recorded on a reel-to-reel tape recorder. Magnetic tape was invented in Germany in the 1930s, but wire recorders were more common in Britain during WWII, having been invented in the USA in 1939. The machine shown, a Wearite 2 speed, 1/4 inch tape deck, was not manufactured until 1947. The episode is set in 1941.
Look closely at some of the freight wagons at 'Limehouse Green' railway station: several of them appear to have 'British Rail Freightliner' livery. This episode is set in 1941, but British Rail did not exist until 1948, and the Freightliner Division did not exist until the 1980s.