The elusive pair of ARMS.
You've already got arms, shitforbrains!
Or whatever term from Jailbreak, Problem Sleuth, or Homestuck you prefer. Newly introduced characters tend to have
no
visible arms, and attempts to grow or retrieve arms result in getting berated by the narrator. You wonder why you didn't see them before.
Humorously, John Egbert quite literally had to obtain his arms from his Magic Chest, although they weren't what one might at first expect. The Wayward Vagabond trumps the player's initial suggestion of retrieving one's arms by interrupting the command to point out that he had them already
, and the command is still given to Jade Harley even when her arms are clearly visible at the time. The Aimless Renegade's command was to quickly retrieve his side-arms, so he picks up his pistols. The Peregrine Mendicant has been the only major character so far, other than the Midnight Crew, to never receive this command.
In the Midnight Crew intermission, Snowman actually did retrieve Spades Slick's arm from a safe.
In some cases, it may be used as a pun where "arms" refers to weapons, but the character responds in the same fashion.
Perhaps the greatest arms gag of all was enacted in A6I5, with John's arm retroactively spread throughout the Homestuck narrative via retcon mechanics. There are 52 (4x13) such arms to be found throughout Homestuck.