Gerald Bryant, Sam's reflection in the mirror, clearly wears a different shirt to Sam's. While Sam's left collar has a brown patch, the collar in the reflection has a white patch in the respective area.
When Sam approaches Donna in the Science laboratory, the blackboard shows Linda as the professor's "assisstant," misspelling the word.
In the café, when Sam is talking with Donna, the Deep Purple poster on the wall appears to be from their reunion tour (or later) in 1984, not from 1972
Whenever Sam (Scott Bakula) is interacting with his (Dr. Gerald Bryant's) reflection in the mirror, the reflection's pipe drops but Sam's does not.
In the cafe, two of the pinball machines were released well after the date of June 12, 1972. The Williams Fun-Fest was released in November 1972. The Bally Delta Queen was released in July of 1974.
Jamie Lee's notebook has a UPC bar code, which weren't available until 1974.