There's the infamous, Narmtacular "What is a man?" conversation at the beginning of the original game.
Or that Evil Elevator? God, Alucard and Richter sound like they're absolutely terrified in it!
Or the "Holy Symbol," an item that lets Alucard go underwater without taking damage: It's a snorkel.
Or the Secret Boots item whose description reads "Discreetly increases height!". If you look very closely (or have a big screen TV) you can see that it actually does, making Alucard a pixel or two higher.
Alucard's recollection of Trevor Belmont, which appears as a thought balloon containing Trevor's unmodified 8-bit sprite from Castlevania III.
In the confessional, if Alucard sits in the Priest's chair, a female ghost will walk up to the other seat. The one in blue will spring a trap just like the green priest, but the woman in red will start giving a tearful confession... And doesn't ever stop. Eventually, Alucard will fall asleep.
For extra hilarity, notice how Alucard sits in a polite way if he's confessing but goes back to slouching with his arms and legs crossed when he's on the Priest's chair, as if he's not even serious about it to begin with.
Summon Alucard's Bat familiar and have him turn into a bat, himself. The familiar falls in love with him, little hearts coming from it, with more bats joining in the higher the familiar's level. It's as adorable as it is hilarious, and they also function as Options that shoot fireballs whenever you do.
Adding to the fun, switching back into human or mist form causes a little "?" to appear over the bat.
There are peanuts as healing items in the game, but their description states they're difficult to eat. Why? Well, this game has an Early Installment Weirdness aspect in that Alucard must equip healing items on his hands to use them outside of the menu, often throwing them on the ground so he can consume them. The peanuts are unique in that Alucard decides to be a show off about it and throws them over his head so the player has to manually move him under the peanut and hold up to make Alucard catch the snack on his mouth. The dhampir even strikes a triumphant pose when the player gets it right.
In terms of the Master Librarian, once the player gains access to both the Gravity Boots (for the High Jump ability) and the room underneath the Librarian's seat....it's possible just for some small rewards (as in, very rare items that can only gotten this way after visiting the Inverted Castle), to push up his seat right from underneath while he holds his arms up into the air out of panic. This is as rewarding as it is hilarious to see him like so, and this is also possible with both Richter and Maria.
Just as funny; the librarian goes on about his business as usual after you stop and return to his desk upfront normally. It may be hinted in that he either didn't give a shit or he had no idea what was going on.
A meta entry, but Igarashi once sat down with Romscout, a Castlevania speedrunner, and with the help of a translator he did a speedrun of the game showing Igarashi both bugs he was aware existed in the game and even a few that he did not. This also brought on some hilarious moments such as when discussing the original bug used to keep the Alucard gear at the start of the game. The glitch was discovered very early on in the game's life and required the game be in luck mode. Not funny on it's own, but Igarashi mentioned that he was the one that programmed Luck Mode in the first place and that seeing the knockback being used to keep the Alucard equipment hurts each time. Some of the other more absurdly difficult bugs had him remark that no bug tester in their right mind would EVER find some of the things Rom was doing.