In the opening sequence on the dock there is a crate with the letters SFO on it. SFO is the airport code for San Francisco International Airport which was not opened until 1927 while the game takes place in 1925.
When Laura looks at the painting in the library before Dr. Feels is murdered, the game comments it is portrait of a bloodhound (maybe Beauregard), pointing out that the dog's eyes look strange. When looking at it after Dr. Feels is murdered, the game message is "A picture is a picture. You see nothing of interest".
After Laura pulls the bell down, the metal crank attached to it is visible. If Laura takes it, leaves the screen and returns - the crank is seen still attached to the bell, although Laura has it in possession and cannot take it again.
According to Laura's notebook, the cigar butt is one of the ten items which require close scrutiny. But unlike the other nine items, examining the cigar butt with the monocle does not reveal anything interesting about it.