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.