It is not a duck, but a goose, in the magic trick.
The sign in front of the Waters farm reads "Water's Dairy Farm." It should have read "Waters'" with the possessive apostrophe at the end of the name.
On the front page of the local newspaper (The Logan City Sentinel), the headline reads, "Mason Will Defend Marvin Adams." Yet, there is an article just below the headline which reads, "Court Seeks Adjournment." (BTW, this article is seen countless times, not just in Perry Mason, but in other '50s and '60s TV series as well.)
When Mason introduces the "newspaper photo" as evidence, since it shows great detail and no halftone dot pattern, it is obviously not from a printed paper, but is an actual photograph.
When Mrs. Adams is found dead of a heart attack, the actress playing her is obviously still breathing.
At 10:52 the white shirted camera crew and gear is clearly reflected in the side of Perry's Cadillac as the gas station attendant cleans PM's windshield.
When Perry and Marv Adams (Gary Vinson) are driving through the desert and get pulled over by the police chief sounding his siren, the view of the front of the police car shows he was not displaying any emergency lights.