Named for the boundary around a black hole from which there is no escape once crossed,
And while I wouldn't necessarily characterize anything going on in the Foobiverse as "evil", I can easily take this concept and apply it to moments where characters became irredeemable; that is, the point after which they can't ever come back and be truly sympathetic again, due to their character being intrinsically linked with their earlier bad behavior. For example:
Deanna may have hit this point when she arranged to have a fake wedding and deceived her parents, or, failing that, where she oops'd Mike on their honeymoon.
Liz may have hit this point with the "WAIT" strip, followed by her sudden bout of homesickness mandating her move back to her parents'.
Mike may have hit this with the fake wedding above, or, possibly, even earlier with his reaction to Deanna's car accident.
Anthony, of course, has "Wait for me! Promise me you'll wait!" although it may have come even earlier.
So where's your point of no return with these characters?