I've of course heard of things happening "at the 11th hour"--very late in a process. But apparently things can happen even later than that:
Alicia Summers of El Mirage, Arizona[, said:] "Did Palin really think she could come into a race at the 23rd hour and not be subjected to questions?"
--Palin daughter's pregnancy stirs strong emotions from iReporters, article at cnn.com, retrieved 2 September 2008, 10:29 a.m.
TSOR suggests that the phrase has been used by others, apparently fairly prominently, in recent years, but it appears to be pretty rare, especially compared to the traditional "11th hour" version. At any rate, this was the first I had seen the "23rd hour" phrasing.