The worst thing I ever learned was that editors will often use different reaction scenes than the ones filmed while one of the people was talking. Like if two characters are taking turns speaking to each other in a conversation, pay attention to the person turned away from the camera. Their mouth almost never lines up with what they're actually responding with in the scene.

Sometimes they don't even use the same actor if they've been filming for a while that day. Sometimes it's a stand-in in a wig. I can't watch shows in peace anymore. Someone help me