I am sad
The "I am sad" fad/injoke started with drak, whose random and crazy comics started many fads/injokes themselves or at least laughs from users on the message board. The "I am sad" craze started with one of his comics that used Matrixx's comic sprites (already used in other comics) and contained Sonic, Knuckles and Amy, along with random, thoughtless dialogue and a mangled reference to Knuckles' theme song from Sonic Adventure. drak made this comic in an outrage after witnessing an absolute trainwreck of a comic strip posted by an unmemorable member of the community in hopes that the aspiring "artist" would learn something from it.
The trend became hugely popular in the Moogle Cavern, especially in other comics. It became "the thing" to rip off pay homage to the strip in your own comic, usually devoting a whole strip to it that was largely the same, though with modifications to the dialogue, characters or some other aspect. Naturally, the became pretty popular in the message board too, and even simple expressions of being down in the dumps, or apathy, could suddenly bring on unexpected quoting.