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.
When Marty first arrives at the Davis homestead and Clarke 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.
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.