Ficverse Fusion, or just fusion, is a phenomenon that occurs when two fics written in the same continuum merge at a point where the Words of the two fics are almost or completely identical. It happens more often than it should because the lack of imagination of most Suethors leads to the use of canned phrasing.
The solution is to quarantine the fics (which means separating the fics, and is unrelated to quarantining the continuum). This can generally be done by activating a portal back to HQ at the site of the fusion, whereupon agents in each fic can return to HQ and from there to their own jobs.
If the fics are not separated, they tend to merge into one fic.
A severe fusion event may have been responsible for the Les Misérables Songfic Crisis; songfics containing lyrics from popular songs are at especially high risk for ficverse fusion since those lyrics are usually the same from fic to fic, give or take a few misspellings.