Spider-man: Bats, I think I might have been married and forgot about it.
Mari-Kari subverts this with a scythe in the elementary school's groundskeeping shed, seen in the second episode. Despite the psychotic ghost slasher girl, it's never seen again.
In the first episode of DSBT InsaniT, 'The Party', Portica brings some strange purple Jell-O...Which turns out to be a remnant of Slima, one of the villains the gang defeated before the events of the web series.
In TOME Gamecrazed receives an important item called the drain edge from Asterob as early as episode two that is able to absorb in-game energy. This was set up so the group could use it for sealing away the Forbidden Power in the finale. An Axe-Crazy computer virus that can hurt the MMO's players in real life.
Parodied in Girl Chan In Paradise when Kenstar points out a bucket of water for no good reason (the bucket is then used to defeat a Bushido Blaster who can set himself on fire. There's also a retroactive one in the same episode, when Kenstar is asked how he knew to break the artist Bushido Blaster's giant pencil, and has a flashback to when he was a kid.
When fighting the antagonist in Dusk's Dawn, Donut and Star Whistle wonder where Breeze Rider is. He shows up later to snatch the villain's weapon.
In Pokemon World Revolution, we get introduced to a race of God-Killers known as The Titans near the end of Season 3. We also find out that Susan.Mageall has no memories of her real parents. Cut to the final season and it's revealed that she is a member of the Titan Race.
In RWBY, rocket-propelled lockers are introduced for the sake of a gag where Jaune gets stuffed into his locker and launched away. In the volume 3 episode "The Battle Of Beacon", they come back into play as a serious way for the characters to retrieve their weapons, as weapons to pin down the Nevermore that's attacking the arena, and as an improvised method of flight for Ruby.
And in the Volume 3 finale, they show up one last time, as Pyrrha shoves Jaune in one to send him away from the fight she knows she is about to face.