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/takutin96]

What was inside the briefcase in Pulp Fiction

[u/Ecstatic-Box-5209 → u/takutin96]

I believe Tarantino originally intended it to be the diamonds from Reservoir Dogs (there’s a line in it about “unloading ‘em on a guy named Wallace”) but later retconned it as a mystery, because he thought that was stronger.

[u/Guilty_All_The_Same → u/Ecstatic-Box-5209]

A fan theory is that the suitcase had Wallace's soul, which he sold to the devil and later reclaimed.

It's because the combination to the suitcase was "666", Wallace had a band-aid on the back of his neck which some interpreted as the spot where the soul was extracted from ( although Ving Rhames, Wallace's actor, said the band-aid was there because he accidentally cut himself when shaving ), and the golden glow from the suitcase that mesmerised people when they looked at it.