(at around 45 mins) When Lex is lying in the hospital he is connected to a heart monitor which reports his heart rate as 60-61 beats per minute. This is the same monitor which is connected to Clark, Lana, and others during earlier episodes, and has the exact same 60-61 BMP variation regardless of the character connected to it.
During the scene with Lana hemorrhaging after giving birth, and Lex being escorted out, we see him pounding on the door and disturbing the blinds, yet in all other shots of hospital rooms in Smallville, the blinds are on the insides of the rooms, not the outside of them.
When Lana dies in Lex's fantasy, the heartbeat monitor doesn't beep for the flat-line.
When Lex splashes water on his face, you can see the bathroom faucet is missing the handle for the hot water.
Lex's mother refers to him as 'Lex' throughout the entire episode, but was known to only call him 'Alexander' when she was alive.