Continuity mistake: During Inspector Kemp's visit at the Frankenstein manor, Frankenstein is playing darts while he is having his conversation with Kemp. He throws the first dart at the bottom 10, but by the time he throws the second dart at 7, the first dart he threw at 10 is now at the bottom of 18. (00:56:20)
Continuity mistake: When Frankenstein releases the monster from the experiment bed, he reaches for the third bar (over the monster's thighs) first, but the fourth bar (over his lower legs) is already open. All of them were closed when they left the monster in the lab. (00:46:50 - 00:53:55)
Continuity mistake: At the castle, Dr. Frankenstein throws darts while talking with Inspector Kemp. The shot of the darts hitting the board does not match with the shot where Inspector Kemp takes the darts back from the board. (00:58:50)
Continuity mistake: During the scene where the blind man, Gene Hackman, is serving soup to the monster, the blind man spills 2 large ladles of soup onto the monster's lap. The blind man then burns the monster by lighting his thumb, thinking it's a cigar. The monster screams, and breaks through the front door, leaving the blind man's cottage. When the monster leaves the cottage, there is no sign of the spilled soup on his clothes. (01:08:50 - 01:10:55)
Continuity mistake: When Dr. F climbs up the hay cart, the first wide shot shows him with his waist above the cart. The next shot, from the cart's POV, shows him only shoulder level.
Continuity mistake: When Frau Bluecher is talking to the Monster for the first time, she puts her hand on his head. In a closeup shot of the Monster's face her hand and arm are missing.
Continuity mistake: When the blind man serves soup, there's a spoon in a cube on the left skewed to the right. A couple seconds later it's straight.
Continuity mistake: The blind man reaches for the cigars, and when the angle changes, half a second later, the broken pieces of the mug have changed position and have also increased and suddenly appear scattered all over the table.
Continuity mistake: As he's about to depart at the train station, Friedrich leans in to try and kiss Elizabeth good-bye. He's mere inches from her nose when the shot cuts, at which point he's suddenly standing more than a foot away from her. (00:16:00)
Continuity mistake: Frankie arrives at the door of the blind man, who welcomes him. Then there's a close-up of the monster from a bit far away where you can see his shirt and head as if he were alone; however, a frame later the blind man is right next to him, his head almost touching the monster's.
Answer: It's called "Transylvanian Lullaby" (as they tell you in the film) and was composed for the movie.
Myridon