A stock shot of George Zucco climbing the temple steps from The Mummy's Hand (1940) is used to represent Yousef Bey (John Carradine), which means that by the time Carradine actually faces the now-elderly Zucco to receive instructions, he appears to have lost about forty pounds and gained a full head of dark hair.
The opening speech by Andoheb is not consistent with the back story given in the other movies of the series. As well, Andoheb died at the beginning of the previous film The Mummy's Tomb (1942).
The High Priest Andohep gives instructions to Yousef Bey saying "During the cycle of the full moon you'll brew 9 Tanna leaves. Kharis will know and come for the fluid which preserves him. In The Mummy's Tomb (1942) it was 3 Tanna leaves to preserve him and 9 to put him in a killing mood.
Most of the Egyptians are played by actors who don't look the slightest bit Egyptian.
When the Mummy is wrecking the museum he has a cut on his chin.
When Kharis crashes through a fence early in the film, the lines indicating where the breakaway fence rails are set to break are clearly visible.
Towards the end, Youssef Bey mouths 'Karnak' but says 'Arkham'
Kharis breaks in to the museum and the night watchman doesn't hear him.
When the Scripps Museum security guard comes into the room to confront Yousef Bey, you can see the shadow of the boom mic on the glass door behind him.
When Kharis slams the museum guard into the plate glass panel, the actor reacts by grimacing and reaching for the back of his head. It appears that hitting the glass really hurt him, but the trouper continues his "death scene" to the finish.