Harry goes through the memory bowl and witnesses a pretty long scene with his Dad, Snape, Sirius and Lupin where they are taking an exam. Afterwards they go down to the lake and the 3 friends start harassing Snape. Harry’s dad (I think it was him, if not it was Lupin) uses the spell that turns Snape upside down so his dirty, holey underwear shows. Lupin doesn’t partipate but doesn’t stop it either. Lily, his mom, shows up and gets them to leave Snape alone. His dad preens for her hut she thinks he is really obnoxious. His dad and Sirius do not come off well – you can totally see why Snape hates them. It really throws Harry for a loop – for the first time he realizes his parents weren’t saints and that Snape actually does have reason to dislike his dad. It’s actually a pretty important scene but I do forget if it was in Book 4 or 5.