Lamarr's lip gloss (but not lipstick) disappears and reappears several times in the kissing scene. Most notably, it is absent in side shots (where it would visibly smear onto Henreid) but in place when the camera is over his shoulder.
Near the beginning of the movie a neon sign for 'Cafe Imperio' is seen in the mirror as a waitress fixes her hair. Despite being a mirrored reflection the sign is (assumed purposely) not reversed. However, it is mirroring the *back* of the sign in a window of the cafe, so it is shown correctly.
During Vincent van Der Lyn's escape, a guard unlocks cell door while being strangled with wire in an impossible contortion behind and below his back without looking.
When an aerial map of Lisbon is shown, it does not look at all like it, and resembles an aerial view of the China territory of Macau and Taipa Island, then under Portuguese administration.
When Vincent van Der Lyn leaves the airport in Lisbon, walking, he is already in Sintra - a different council, some 50 km away.
The aerial view of "Lisbon" shown as Henreid narrates his arrival there is actually a night view of the Port of Oakland, Yerba Buena and Treasure Islands in San Francisco Bay, California.
During his prison escape, Victor walks into the guards' room in disguise with the large ring of keys in his hand and leaves through another door. It is more likely that a guard would have hung the keys up in this room for the other guards to use as needed and that the guards present would have reminded him of this.
This is actually both a plot hole (shared with many other movies of this genre) and an anachronism, but with only one choice available I'll lodge it under "Plot Holes."
When Vincent van Der Lyn is arrested at the site of a murder with the murder weapon gun in his hand, a gunshot residue test had been available worldwide since 1933 which would have easily cleared him. The plot hole is actually two plot holes, first that someone who finds a freshly-killed or nearly-killed person just happens to pick up the murder weapon, which everyone knows not to do but yet is an essential plot element in the mystery noir/gumshoe and international mystery genre of the era, and second that since 1933 gunshot residue tests are available to clear anyone entrapped in this situation (which is almost reverse anachronistic since it's the NON-use of an older existing and commonly available item instead of the use of an item from a future era.
When Vincent van Der Lyn is arrested at the site of a murder with the murder weapon gun in his hand, a gunshot residue test had been available worldwide since 1933 which would have easily cleared him. The plot hole is actually two plot holes, first that someone who finds a freshly-killed or nearly-killed person just happens to pick up the murder weapon, which everyone knows not to do but yet is an essential plot element in the mystery noir/gumshoe and international mystery genre of the era, and second that since 1933 gunshot residue tests are available to clear anyone entrapped in this situation (which is almost reverse anachronistic since it's the NON-use of an older existing and commonly available item instead of the use of an item from a future era.
When the first conspirator is shot in an alley, a policeman identifies the dead man's gun as "7.63 Luger." Lugers are 7.65 mm; the 7.63 is a Mauser, of a different cartridge length. They don't look alike.
The two cops in uniform that escort the police detective in the restaurant after the opening murder, are wearing black top hats similar to those worn by English bobbies then, which were never part of the Portuguese police force. Some twenty years later, Portuguese traffic cops would use those top hats, but in white, when directing traffic in busy crossroads.