When Poirot and Race are interviewing Tim Allerton on deck, the shots of Poirot and Race both show the bank in the background moving slowly right to left. But in the close-ups of Allerton the bank is not moving.
We see Poirot leaving what is a very spacious cabin for on-board ship. As soon as he is outside the door, we see that the adjacent cabin door is very close. That cabin must be like Doctor Who's Tardis - much bigger on the inside than on the outside.
When the head of a stone statue falls off the side of the temple and almost kills Linnet, it bounces when it hits the ground. There's no way something made of solid stone would bounce. It should either stay completely still or crumble when it hits the ground, but the visible bounce reveals that it's not made of stone but of something much lighter, like foam.
When Salome Otterbourne is shot, her dress briefly opens as she falls, and the actress makes a rapid movement with her arm to cover herself.