Continuity mistake: In the scene where Inga, Fredrick and Igor throw the net over the monster, Inga sticks the syringe of the sedative in the monster's back but does not push down on the top, yet the monster still falls over. A few seconds later when she collapses on the monster, you can see all of the fluid inside spray out. (01:12:05)
Revealing mistake: When Helga is sent flying into her room, the support cable that is attached to her is visible.
Audio problem: When Frederick is playing darts with Inspector Kemp, the last dart that Frederick holds flies backwards when he raises his arm quickly causing the dart to land on the floor, but when he throws his now empty hand forward, the sound of the window breaking is heard.
Revealing mistake: One of Peter Boyle's teeth on his lower jaw was painted black for that "broken grin" look. It can be seen reflecting in some scenes: when Frankenstein enters the cell with the monster and it cries, and later when the monster screams while killing the jailer. (01:15:30 - 01:28:40)