When you first pass through, there are two special Bone Scimitars in the large room where you find the first save point. One will always drop a Short sword, and the other will always drop Red Rust, even if you save and reload and kill them again. When you return later, they will be replaced with Bloody Zombies. Earlier enemies and even some candle contents will also change.
Once you gain the ability to turn into a Bat, you can fully explore this area and find a few more items.