Throughout the movie, there are several times where the couple places logs in the fireplace and starts a fire. During the last one, George starts the fire and places the screen back in front of the fireplace. Within seconds, the fire is blazing strong.
The morning of the first encounter, Doris goes into the bathroom to take a bath. The sheet she has wrapped around herself keeps changing positions. It covers both shoulders when she is outside and only one when she is in the bathroom.
The exterior of the cabin set (window and door placement, size, etc.) does not match the interior set.
The intervals are mostly five years, except after 1966 it skips to 1972, as this marks their 21st anniversary. However, in the 1977 meeting, George brags that he saved Doris and Harry's marriage "six years ago" - in 1971, not 1972, which was the actual meeting in which George pretended to be a counseling priest.
George tells Doris his watch is three hours and 25 minutes fast. But when the watch says 12:00, it's actually 8:25, which means it's three hours and 35 minutes fast.
In the 1966 sequence, George refers to voting for Barry Goldwater because his son has just died in Vietnam and says that they received the news at the most recent 4th of July sometime in the last few months. Goldwater was a Presidential candidate in 1964 - two years before. He would have voted for Goldwater two years before his son died.
When Doris is in labor in the bathroom and gives her doctor's phone number to George, a boom mic is visible for about 15 seconds.