Retroactive continuity, or retcon for short, is the alteration of previously established facts in the continuity of a fictional work.
Retcons exist in the Tom Clancy's Splinter Cell series because of multiple writers in the various forms of the Splinter Cell media that change already established storyline details, plotlines, character backgrounds, etc. In some instances, the developers may address this issue while some details have not specifically been talked about. In these cases, it may become unclear what is established and what isn't because of inconsistencies (in the specific areas, these will be addressed).
Third Echelon Edit
In Tom Clancy's Splinter Cell, the very first level "Training" (taking place August 7, 2004) reveals that Third Echelon was a brand new initiative following the close of Second Echelon, when the National Security Agency experienced it's first system-wide crash on January 22, 2001. However, the novels place the founding of Third Echelon before 2003, and Sam Fisher is already a member before 2004 (the time the games show him recruited). Tom Clancy's Splinter Cell: Essentials also contains a mission that is set during 1999 that shows Third Echelon operated and Sam Fisher was a member. The game also retcons Sam Fisher's first meeting with Anna Grímsdóttir in the 1990s, whereas he first met her in 2004 (in accordance to the games).
Also, in the early Splinter Cell games, Third Echelon in a secretive sub-unit of the NSA, but in Conviction, it's shown to be a pretty public agency of its own.
Sarah's 'death' Edit
In Splinter Cell: Double Agent (Version 1), Sarah Fisher's 'death' was shown to be in 2007, but the cutscenes of Splinter Cell: Conviction place Sarah's death in 2008, seemingly confirming that at least that element of Version 2 of Splinter Cell: Double Agent is at least canon. Splinter Cell: Essentials also states that Sarah's death was actually in 2008, whereas Version 1 says it was in 2007.
In Splinter Cell: Essentials, much of the story has elements that are canon, while much of the elements are considered non-canon. For instance, Irving Lambert's death by Fisher's hand is confirmed in the story, and accepted as canon (as it is). However, the game states that Fisher killed Hisham Hamza, but this is not canon. It has never been confirmed that Essentials in canon, as much of the story points don't line up with the rest of the series as well.