Tom and Karen are on a lighthouse situated on rocks when triffids appear. Tom turns a fire hose on them spraying the with salt water which causes them to melt in which case hoe did they survive the spray from the waves crashing on the rocks.
The force of water from the hose Tom and Karen later use to destroy the triffids is much greater than what the triffid would have been subjected to by sea spray; when Karen told Tom about the triffid being on a rocky ledge and they returned to look for it, they were not soaked by the sea spray, so evidently the triffid would not have been either. As seen earlier in the film , the triffids grow incredibly quickly, so would only have been there for a few minutes when Karen saw it. In the short time she was away, the triffid moved away from the danger.
When Bill is about to dive into the sea to be picked up by the rescue boat, he tosses his cap away onto the cliff, but at the end of the film, when he and the others are climbing the stairs to the church, he's once again wearing the same cap.
At the villa in Spain, when the triffids are attempting to get through the electric fence, the action goes from bright sunlight to darkness to evening and back again.
When Howard Keel sees the rescue boat, he waves his hat to get their attention. When he jumps off the cliff, he throws his hat in the air, yet, when he is shown on the sub, he's wearing his hat.
When the security guard is killed, you can see the triffids moving towards him on wheels. They are clearly visible for a couple of seconds.
The morning after a Triffid attack, Bill is walking back and forth in front of one of the vehicles, and the smoke moves with the camera, showing it to have been overlaid and not actually present.
The Navy broadcast that the sub is leaving but the lifer raft will continue to search for survivors. The life raft is revealed to be a tiny inflatable dinghy with no means of propulsion other than oars and no radio. Anyone picked up this way is almost certainly worse off than they were before.
At 51:00, Bill is dissecting the plant when he says, "and they don't usually pull themselves out of the ground and chase you." Portions of this line are dubbed in as his lips don't match the words and you can clearly hear the dub.
In the final shot of triffids approaching Bill driving a truck, the operators' sneakers can clearly be seen walking the triffid props towards camera shot.
The wires pulling the triffids along the ground can be seen, especially in the scene were one of them chases Susan.
At one point, Susan is in the van and Bill asks her to open the door but she can't. He eventually smashes the window with the butt of a machine gun, reaches in and easily opens the door, without even needing to unlock it.
After Tom & Karen on the lighthouse discover the Triffids can be destroyed by sea water how do they get the news out? They haven't got a boat to get back to the mainland and the radio they have isn't a transmitter.
Teresa is going to give birth within a day, but she's as skinny as a rail.