The snow on the ground is obviously sand, this is noted when Mr. Teppler is digging to open his underground safe.
The murder happens on November 15, but the double-month display calendar on the wall where the murder occurs is turned to July/August of 1959.
When various phone calls are made, only 4 or 5 digits are dialed. UPDATE: This was common in smaller towns at the time, for calls within the same town. The phone in Perry's motel room has no dial at all, made for incoming calls only.
At the end of the show, all are standing in the snow-covered woods, but you can't see anyone's breath. UPDATE: The air temperature could be warm enough so that a person's breath would not be seen.
Mason is in Tepper's store to question him. All of the merchandise on the shelves behind the counter is upside-down. An upside-down box of Ritz crackers is clearly visible.
Sheriff Norris investigates his own daughter's murder. This would never be permitted for obvious reasons. The San Bernadino County Sheriff's office would have been called in to perform the investigation.
Sheriff Norris tells Perry Mason that Phil Beecher has stolen the "bimonthly" payroll, when he means "biweekly."