When Harold imagines his encounter with his "Vampire", he ticks off the experience in a memorandum book on the verso side, the recto side of which lists the month as October. When he imagines his subsequent encounter with his "Flapper", he ticks off that experience beneath the previous one, only the recto side of the book now reads as April.
When Harold helps Mary out of the boat, the lilting boat is causing a lot of ripples in the water behind them. In the very next shot, the water has become still again, with none of the wake from before.
It is implied that Little Bend does not yet have electricity, since Harold is shown using a gas-fueled wall-light and oil-lamp, and the clothes-iron heater in the shop uses a gas-burner. Yet many utility poles are clearly visible in later shots of the downtown area.
When Mary's car goes off the road and in a close shot she takes out the crackerjack box, there is a reflection in the side of the car (bottom left) of a pair of legs standing nearby, then walking away.
Harold rips the publisher's envelope into small pieces (and indeed, he tears it into such tiny shreds that the check inside is torn into four narrow strips at one corner, so that the digits of the "3000" are separated into individual shards of paper), yet the cover letter stating, "Dear Sir, We have decided to publish your manuscript as a humorous work and have renamed it 'The Boob's Diary'. We think it's very funny. Enclosed is a check for $3000 in advance royalty payment" is mostly undamaged, with just a single straight tear across the middle of the sheet. If a sealed envelope is torn up into that many pieces, then the entire contents will be shredded, also --- one enclosure would not get torn up finely, and yet another paper be able to escape with just being torn in half.
In the principal of "united we stand" (as is so eloquently illustrated with the bundle of sticks in "The Straight Story"), a thick sheaf of paper sheets is hard to rip through all at once, so it would be impossible to tear up several layers of folded paper into narrow strips, the way Harold supposedly does with the check when he tears up the sealed envelope without opening it. To actually tear the check between each of the digits of the "3000" so that it formed four individual narrow strips, Harold would have needed to remove the check from the envelope, and also separate it from the cover letter, so that he could have just the single fragile sheet to minutely tear into the thin shards.
The movie presents Ronald DeVore as a selfish status-oriented man who would probably insist on Mary's wearing her marriage rings all the time, yet in the scenes of Mary being assisted in preparing for her wedding and of her standing with Ronald at the alter, she is not wearing an engagement ring.
Harold has to turn to his right to drive down the rutted detour route, yet when Harold re-enters the highway a minute later, the alternate route's exit is on his left, not his right. Obviously just a second shot of the same detour-entrance with its long line of traffic barriers, filmed from the opposite direction.