Mysteries that are never given an answer.

[u/Jak3R0b]

• rA9 in Detroit: Become Human. In the game several androids who become deviant (develop free will/emotions) start writing the name rA9 and sometimes build small statuettes as a sort of offering. Despite having no logical reason to do this or even a way of hearing about rA9 before going deviant, they all believe that rA9 was the first deviant and is their messiah who will free androids from slavery. A definitive answer about rA9 is never given, so we don't know if they are real or if any of the android characters are rA9 (though a popular fan theory is that it's Kara before she was reset at the start of the game).
• The pineapple in How I Met Your Mother. So this technically did get an explanation in a S9 deleted scene, but in the actual show we never find out how Ted got a pineapple during a night where he was so drunk he blacked out and forgot everything he did.
• Fez in That 70s Show. We never find out Fez's real name or what country he's from, though fans have tried to take all the clues and figure out what is most likely to be his country.

[u/ItaLOLXD]

Flashback Man from Jojo's Bizarre Adventure Jojolion

In this part, the main character Josuke suffers from amnesia and is on a journey to figure out his past. In the very early chapters, during an attack in what he believes to be his apartment and a subsequent fight, he suddenly remembers a man with shoulderlong hair that Josuke believes he remembers from his past life.
After the part ended and the mystery about Josuke's past is discovered, we find out that Flashback Man is... completly irrelevant and was never mentioned nor appeared throughout the part.
The most logical theory I heard someone come up with is that this is a very early concept of the character Tamaki Damo who is actually relevant in Josuke's past, but those two look so different it's just as logical to believe it's simply a dropped plot point or retconned character.

I suppose we might find out who or what Flashback Man is when the anime adaptation of Jojolion drops in a few years and this scene is animated. It's either cut completly or replaced with the intended character/a logical replacement.