Neville's death. Harry, Ron, and Hermione bully Neville to the point where Neville self-destructs. Not funny. But then they begin eating his remains, and it's hilarious.
Puppet Draco's death too. Draco getting decapitated and set on fire by Harry is a little too mean-spirited, but Draco getting decapitated and set on fire by Harrywhile Harry sings a happy song about what Draco likes? Hilarious.
Some of the entries in "Snape's Diary" are really quite sad. But the dry, deadly serious delivery of lines like this make it impossible to watch with a straight face. (The organ playing in the background and the fact that Snape is lit by a single candle doesn't help.)
Today I ate some oatmeal for my breakfast.It was flavorless and watery. I thought of my mother. I cried.
"Funny Aneurysm" Moment / Hilarious in Hindsight: Depending on your point of view, "Stopper On My Pain", a song about Snape's "lost love", is probably one of these (Or a mix of both). The song, written long before Deathly Hallows, was actually supposed to be a melodramatic ballad about Gilderoy Lockhart. Through pure coincidence, it almost perfectly describes Snape's feelings for his real lost love, Lily.