When Marian saves Davey from drowning towards the end of the film, she pulls him out of the water and her hair is soaking wet. Moments later, it almost dry & is scraggly.
The gray in Marion's hair has completely vanished when she and Ben are out on the porch at night and she has the tea tray.
When Ben chases Marian after they both emerge from the pool one night, suddenly Ben's swim trunks are dry.
Early on in the movie Ben's (Oliver Reed) glasses appear and disappear from his face in a couple of scenes, without him putting them on or removing them.
When Ben is kissing Marian on the lawn, after they emerge from the pool one night, blue eyeshadow appears & disappears on Marian's face.
The Kirby vacuum that Karen Black is using on the steps, she turns it off via a switch on the handle. That series of Kirby never had a handle switch, it would have had a toe switch down by the motor.
The Rolfs never actually pay the $900 rent for the Summer.
Ben (Oliver Reed) is seen chopping bushes in the garden midway through Burnt Offerings. When he pauses for a rest, he suffers a terrifying waking nightmare as a black hearse drives up the road to the house, pauses, and the phantom chauffeur smiles at him insidiously. The delusion then ceases abruptly! Yet, although the black hearse is supposed to be unreal it leaves a small oil leak on the concrete path where it was previously parked, after it has vanished.
When all the clocks move to midnight on their own, wires are visible pulling the hands around.
At the end of the film, when the chimney collapses, you can clearly see the cables on either side pulling the bricks down.
There is no clue or significance in the movie of the prop antiquated bicycle found in the woods, or the Edwardian/Victorian spectacles found in the pool. It is never actually specified, for instance, that they have any relationship to the 1890s graves. Indeed Ben's (Oliver Reed's) sudden bout of madness upon discovering the cursed glasses in the pool, could have played identical, with or without the spectacles.
When escaping from the renewing house with his son, why does Ben relentlessly plow his automobile into a fallen tree? Surely, it would have made more sense to drive an alternate escape route; or even abandon the car altogether and escape on foot.
It is inexplicable that there is never any police investigation and arrest, given that dozens of people have vanished when visiting the country mansion (Dunsmuir House). The Rolf family, for instance, having recently attended Aunt Elizabeth's funeral, must have sent out invitations or made phone calls from the mansion informing relations, shortly before they too disappeared. The doctor was also aware that the Rolfs had leased the mansion. Leading the authorities to suspect the Allardyce siblings and Walker of foul play.
At the end of the movie, as they are preparing to leave the house, the shadow of a boom mic is clearly visible as the family walks around their car.
Just before Ben finds the broken glasses, and gets possessed in the pool trying to drown Davey, he misspeaks and says "You know, Benji, you may be a great ball player, but you float like a rock, understand?" He should have said Davey's name, not his own.