When West and Gordon enter the card shop, it is night time (dark outside). When the henchman is shot through the window, it is very light outside (we can see the shadow of the shooter on the glass). When West and Gordon run out of the store in pursuit of the shooter, it is night again.
The piano wires in the chamber above the wedding room are so loose they could not possibly be making any musical notes.
In the first scene aboard West and Gordon's rail car, the window on the right has mismatched backdrop images in it.
As the Wedding March starts playing, the overhead shot of the hammers and strings reveal that the music being played doesn't match the strings being pulled. Additionally, when Elaine is shown playing the piano, the notes don't match her fingering.
In the final fight, after the laughing henchman face plants in the cake, West hits him in his face, which is covered with cake and frosting - yet West's fist is clean after the supposed contact with his face.
P. J. Lambert and Michele were already officially pronounced man and wife during a wedding ceremony, however at the end of the program they sent a wedding invitation to Jim and Artie even though their first marriage was not annulled.
When West and Gordon begin interrogating the bald henchman in the print shop, he is shot in the back by someone outside the store. The shooter was able to shoot through a stained glass window (which was only translucent, not transparent) and actually hit the henchman in the back. Pretty good shooting for not being able to see through the window to know where the target is. Also, with as much light as is obviously shining through from outside the store, the shooter would not even have been able to discern shapes through the translucent windows.
The 2 cards where Jim and Arte discover the flawed serifs had the letter M 3 times, but only the 1st and 3rd were flawed. Note that it is highly probable that the printer would have a standard die for the 'Mrs.' salutation (which is the only place where the 'M' is flawed). Also note that in the invitation that Gordon finds in the print shop, none of the 'M's (in 'Miss', 'Mr', 'Michele' or 'Master') are flawed. The problem here is that while West should be looking for the 'Mrs.' die, he still insists (later) they are looking for the 'M' die, despite all evidence to the contrary.
During the final fight sequence right after the laughing henchman goes face first into the cake, Henry Beckman (Paul J Lambert) is seen in the background left breaking character, laughing out loud and looks at Ross Martin to see his reaction before getting back into character (looking serious) the next time he is shown in the frame.