Jonathan picks the lock on the boot using only a lock-picking tensioner. He would also need a picking device to adjust the tumblers.
They managed to steam off postage stamps with a constant stream of steam, using a modern kettle - which normally would switch itself off once boiling, though it could be possible that they weighted the switch so the kettle would remain on.
Towards the end , when a hole is made in the brick wall, and reveals the dead killer. The killers eyes are open and look very much alive. Even though after 2-3 hours after death, the killers eyes would have clouded over.
During the reconstruction of the murder, a fraction of a second before Jack Holliday's body hits the ground, he reaches out to break his fall, in spite of the fact that he is supposed to have died instantaneously.
When Maddy leaves Jonathan's room at the hotel, a foot from one of the crew members can be seen entering shot behind her.
The outlet on a toilet is not directly below the pan, but the cistern, meaning there was nothing really unusual about the position of the hole.
Maddy removes the knife from the melon with her bare hand. Being a seasoned investigator she should have known this would cancel any chance of determining the fingerprints of who put the knifed melon on Jonathon's bed.