Retroactive continuity

Retroactive continuity is the deliberate changing of previously established facts in a work of serial fiction. The change is informally referred to as a "retcon," and producing a retcon is called "retconning".

The Resident Evil series has suffered "retcon" on multiple occasions.

t-Virus v. T-Virus
Originally, the main mutagen in the meta-series was referred to as the T-Virus. However - since 2002 - the main series has altered the spelling to t-Virus.

The new spelling has been used in Resident Evil Zero, Resident Evil: The Umbrella Chronicles, Resident Evil:Degeneration and Resident Evil 5.

Barry Burton
The character of Barry Burton - according to the epilogue files of Resident Evil 3: Nemesis - survived the events of the Mansion Incident. This is apparently due to a merge with certain events in Resident Evil, in which one character is given certain event attributes.

In the Epilogue File, he is said to have sent his family to Canada before joining Chris to fight Umbrella in Europe.

The release of Umbrella Chronicles, along with an apparent interview retconed his survival.

When playing as Jill in the remake of Resident Evil, keeping Barry alive causes Wesker to be thrown at the wall of the lab - rather than being impaled which UC shows. Wesker manages to escape during the first fight against the Tyrant T-002 (off-screen) and so a file which mentions the G-Virus is not obtainable.

In Resident Evil 2, a file is shown in which Chris sends for information on G to be released. By Chris pocessing this item without Barry's apparent survival, then Chris's scenario is the canon one of the game. This, along with UC, shows Rebecca Chambers' survival. The BioHazard Archives say that she is dead.

These, among others, have led to public disputes over which canon source to trust.