Common mistakes in writing sex scenes from novels include over - focusing on the physical details rather than the emotional connection between the characters. This can turn the scene into something that reads like a medical report. Additionally, not pacing the scene correctly. If it's rushed or too drawn - out, it can disrupt the flow of the story. Also, not being true to the characters. If the characters' personalities don't seem to match their actions during the sex scene, it can be jarring for the reader.
One common mistake is being too explicit. It can make the readers uncomfortable and might even seem like cheap thrills. Another is not connecting it to the characters' emotions. A sex scene should be more than just a physical act, it should tell something about the characters' relationship. For example, if it's just described in a mechanical way without any feelings involved, it's a mistake.