"Character rupture" refers to a canon character's status as extremely OOC, usually anywhere from 60-99% on a CAD reading, though the danger starts at 50%. Once a character reaches 100% OOC, they actually cease to be that character and are instead a character replacement.
Character rupture is a sort of quasi-psychosis affecting canon characters in the presence of a Mary Sue or other authorial influence. A canon character suffering a character rupture has nearly or completely lost touch with his or her original self. Such extremely OOC characters present a serious danger to the integrity of the continuum. Could Frodo have destroyed the One Ring if he were busy gettin' it on with Gollum? Could Harry Potter have defeated Voldemort if Hermione were too busy getting a make-over to translate the tale of the Deathly Hallows? We don't think so.
Pointing a CAD at a character in character rupture has a high chance of damaging or destroying the CAD. Thankfully, experienced agents can often tell when a character rupture has occurred without using a CAD.