When Mac is looking over Betty Parker's diary with Penny, he is holding a mostly-eaten Popsicle with about two inches of frozen treat left, yet in the next shot as he prepares to leave for town to buy forensic supplies. he is holding an empty stick; he and Penny had been sharing a running conversation all the while, and so there was no lapse of time between shots for him to have eaten the rest of the Popsicle.
Penny removes the dust covers from the portraits, but then a few scenes later, when she investigates a noise and looks down the hallway, the dust covers can clearly be seen back on the portraits. And then in the following scene when Penny runs down the stairs, the dust covers are no longer on the portraits.
The disinterred skull changes position between shots, from lying on top of the dirt to being half buried.
The front door of the mansion stands open without any door-stop after Mac and Penny first open it and go in, yet it springs shut in the next scene, and so Mac has to prop it open with the chair.
Betty Parker could never have ingested/absorbed so much lead that her bones would become metallic enough to actually spark when scraped with a knife. She would have been poisoned to death long before the lead content in her bones reached that high a level.
Lead does not visibly spark when scraped with a knife, especially if it is in a "non metallic" oxide or sulfate form.
There is obviously nothing wrong with the seatbelt latch, Sheriff Howells merely does not operate the release-button properly. He clearly is just yanking on the buckle without pressing the button at the same time.
Thunder is heard after Mac goes out again to get supplies from his jeep, yet the windows are glowing with bright sunshine from outside.
Penny tells MacGyver in her first appearance that "Penny Parker" is not her real name, but in this episode, it's represented as her real name. The last name obviously is the same as her aunt's, and she mentions that it is her own name (Penny) on the cover of the diary.
All the events are suppose to take place on Penny's 25th birthday. However, the first time we meet Penny (Season 1 in Every Time She Smiles) her passport is taken which lists her birthday as Feb 20, 1962, which would mean her 25th birthday would be Feb 20, 1987 and not (as is shown in this story) Oct. 31, 1988.