If you use the Plum Flute item against the Papa Plum boss from Boss Butch, it will spawn an enemy Baby Plum! Killing her will spawn the Reinforced Plum Flute item, which summons Papa Plum to pummel your foes!
A correction from a developer of the mod has revealed the true purpose of this backdrop; its existence as a bonus Prank Shop room accessed via Red Key!
(Thank you, fly_6!)
In the files of the Boss Butch MOD, there is a file called "dont open.txt" which contains a somewhat rude phrase and a slightly malformed Joker from Balatro.
Some of you may know that Oops! has a 1% chance to apply the Kamikaze effect on Isaac after 20 uses. A lesser known fact would be that after this chance is applied 3 times, there is a 50% chance for the next application to be a Mama Mega effect instead.
While Great Gideon cannot appear in the Grotto floor from Fall From Grace, there is a rare Plumpod II boss room with a "Wall Gideon". It will spawn an item if you manage to kill it with the Chaos Card. 🪨
Using the the G6 on "A Pony" or "White Pony" will turn it into a special green item called Scallion. It gives flight and a speed increase, similar to the pony items, but it's effect is quite different.
The Fairy Flies spawned by the Lost Artifact rock trinket have unique, randomly-generated names! The rarest of them all, however, is the ending part "Featuring Dante from the Devil May Cry series."
Fiend Folio has some exclusive Dip familiar variants for the new poops it adds! There are even Spider-Dips and Bee-Dips that can be spawned from Beehives or Nests.
There is an extremely rare rune in the Fiend Folio MOD called "Ansus". It has a 1 in a million chance of replacing a rune, and it activates the Amnesia pill effect.