In one scene, Clark mentions "finding something in the icebox". Although the icebox was invented in 1840, it would not have been used by pioneers like them. The icebox relied on a regular delivery of ice to function. The iceman did not cometh.
When Marty first arrives at the Davis homestead and Clark offers her some ham, as she picks up a piece it can be seen that the piece is spiral-sliced ham. The invention and patenting of the spiral slicing method for ham wasn't until 1949 by the founder of the Honeybaked Ham Company.
At the end, when Clark goes after Marty, he's riding his horse. When he brings her back to the homestead, they're in a wagon.
A reviewer claims an "ice box" didn't exist at the time of the movie. The first saving of ice was in 1700 BC! During the pioneer times, ice was saved in ice houses using saw dust for insulation.
During the scene where Clark proposes to her in the rain, a bra strap is visible underneath her wet clothes. Bras weren't invented until the 1910s. A corset would have been worn.
The covered wagon in the opening shot has safety chains that are chrome-plated--a process not invented until well into the 20th century.
When Marty follows Clark into the hills (around 48 Min's)and watches him singing a hymn, the trees she watches from are in full leaf as if in summer, however on the way back to the cabin she is caught in a blizzard and has to be rescued by Clark.
Aaron Claridge's eye blinked/twitched when Marty was looking at his dead body hanging over the horse.