Continuity

When Louise strips before she jumps into the pond, she has on white underpants, then when swimming she has on black underwear. After she gets axed at the edge of the pond and dragged away, she again wears white.

Boom mic visible

When Billy and Kane are walking around the pond, the boom mic falls into view. A little later, the shadow of the boom mic operator is visible on Kane.

Continuity

In the movie's latter half, Dr. Caleb enters a work shed and discovers the body of Louise and the lifelike Kathleen doll. Carefully picking up the doll, he closes the shed door, and immediately in the next scene is shown walking with the doll in his arms and a cigar in his mouth.

Continuity

In some shots Ron Perry as Arthur has two good arms, but in other shots his left arm is in a cast.

Continuity

At the wedding, Caleb removes his tie whilst talking to Kane. His shirt alternates between remaining buttoned to the neck and being open at the neck between camera angles.

Continuity

As Kane and Richard leave the wedding reception, she has her veil on. In the next scene as she approaches the barn, the veil has gone.

Character error

When Billy and Kane are talking by the pond, Billy mentions that they always hold black umbrellas, but at the funeral Lady Haloran and Billy are holding black umbrellas, however Richard is holding a white umbrella.

Continuity

When Billy is holding the candle whilst talking about being stressed, the candle has been smoldering quite a bit of smoke and the next shot shows the candle is lying on the table and there is absolutely no smoke.

Continuity

When Louise goes into the pond after stripping to bra and panties, her panties are tan or light brown, yet in the underwater sequence they are black.

Miscellaneous

Most references to the castle refer to it as "Castle Howard" (which is the name of a real place, but in England, not Ireland), but on a couple of occasions it's called "Castle Haloran" instead.