Retcon mechanics

Retcon mechanics are the result of a unique ability imparted to John Egbert upon his touching of The Treasure, which enabled him to transport himself throughout the narrative of Homestuck, and selectively re-write the metacanon.

While at times these mechanics have far-reaching retroactive effects on the story, granting the bearer of the retcon ability (thus far restricted to John) powers equal to or even rivaling that of the author, the precise scope of these effects seem to vary. Possibly at the discretion of the user (or perhaps from author intercession to prevent things getting too confusing for archival readers), retcons instigated through this effect vary in scope from complete and seemingly irreversible modification of previously published canonical content in the real world, to simply re-writing the internal history of the narrative, most significantly in the case of the retconned Game Over timeline. Often, these effects intermingle, resulting in a high level of Weird plot shit that may be confusing for even seasoned readers to parse. As John himself says HS 8320,.

It should be noted that in instances when the past is altered via retcon mechanics, even if this only effects the chronological perspective from within the narrative rather than changing the meta-narrative, i.e. the actual structure of the comic, the timeline is altered without resulting in the previous reality becoming an offshoot doomed timeline. In fact, reality is entirely altered, with the post-retcon reality consisting a new alpha timeline, and the previous alpha timeline paradoxically erased from history. However, potentially due to the far reaching power of the Furthest Ring to reclaim souls that are otherwise Void, characters who died in the pre-retcon timeline can still exist within the Furthest Ring afterlife. This is the case with pre-retcon Vriska ((Vriska)).

Origin
The retcon ability imparted to John might have been the promised power granted by the treasure sought by Vriska throughout her journey through the dream bubbles in Act 6. As the weapon was promised as a means to defeat Lord English, who possesses unconditional immortality that can only be overcome through exploiting glitches in paradox space, the ability of retcon mechanics to literally re-write the narrative even from the perspective of the readers might constitute a "glitch" powerful enough to erase or otherwise remove Lord English from history. This would also have implied that no character other than John would be able to exploit retcon mechanics, except perhaps Andrew Hussie, who has HS 6713 (has already been killed), as the Treasure can purportedly be used only twiceHS 7952.

However, Vriska believesHS 9749 that the offensive capability of the weapon has yet to be used against English, and if true this implies that the retcon abilities may have simply been a side-effect of touching the Treasure. This means that additional characters might similarly be able to gain this ability.

Arm Retcon
Upon first encountering the treasure, John placed his hand into it HS 7994, scattering instances of his arm throughout the Homestuck narrative. This retcon affected the real meta-narrative of Homestuck; that is to say previous pages were amended in the real world to include John's arm, where it had been brought into being by a real "retcon". There are 52 (4x13) such retroactive appearances of John's arm throughout Homestuck, which are all HS 8004 (revealed) HS 8005 (on) HS 8006 (these) HS 8007 (pages).

John's Random Retcons throughout the Narrative
Shortly after gaining his retcon ability, John discovers that he is unable to control it, and finds himself seemingly randomly HS 8009 (transported) to different points within the Homestuck canon. This results in HS 3295 (various) HS 5957 (incidental) HS 7180 (modifications) to many previous panels, another change to the meta-narrative. It should be noted that in instances when the past is altered via retcon mechanics, even if this only effects the chronological perspective from within the narrative rather than changing the meta-narrative, i.e. the actual structure of the comic, the timeline is altered without resulting in the previous reality becoming an offshoot doomed timeline. In fact, reality is entirely altered, with the post-retcon reality consisting a new alpha timeline, and the previous alpha timeline paradoxically erased from history. However, potentially due to the far reaching power of the Furthest Ring to reclaim souls that are otherwise Void, characters who died in the pre-retcon timeline can still exist within the Furthest Ring afterlife. This is the case with pre-retcon Vriska ((Vriska)).

Later, as John's transportations start to become less scattershot, he alters the past by appearingHS 8302 in front of Dave Strider and Jade Harley whilst the latter is controlled by the Condesce, avertingHS 8304 a confrontation which previouslyHS 8288 had resulted in the Wayward Vagabond being pushed off of the building that they were standing on. Unlike previous retcons, this did not affect past events in the meta-narrative, and the previous run of events still exists within Homestuck. In the resulting conversation between John and Dave, it is confirmed that this retcon has in fact modified the alpha timeline.

Even later on, a second instance of John appears from the future, and prevents the version of John who had previously changed the course of events in the Dave/Jade conversation from being able to do so (see below).

Oil Retcon
Following Game Over, John resolves to visit Typheus with the hope that his Denizen will be able to, which also puts him on the path to completing his personal quest. Before he leaves LOPAN, Terezi Pyrope tries to use her Mind abilities to give John some guidance in using his retcon powers, but it apparently fizzles outHS 8949. John then travels to the center of LOWAS, where Typheus HS 8987 (floods the core with oil), forcing John to choose between retcon-zapping himself away or drowning in oil. Instead, John manages to pick a third option, zapping the oil away itself, to throughout Homestuck canon. This marked his, and gave him full control over the use of his retcon abilities.

Once the oil is gone, John plays, blowing away the clouds on LOWAS, zapping the entirety of LOWAS out of "canon-space", and blowing the stardust out of the Act 6 Act 6 Supercartridge Expansion PackHS 8998.

John's Controlled Retcons
After an intense staredown HS 8320, he sends the previous John away to find Roxy, annulling the effects of the retcon that his past-self had instigated. (Note: discussion of how this creates a "stable retcon loop"/John retroactively retcons into his own timeline - should be analysed in this section as per John's own discussionHS 8320 on the subject).

Game Over Retcons/Terezi's Instructions
(Table of passwords)