Thanks for nothing, leap seconds.
While 'necessary' to keep TAI coordinated with the sensory experiences of the inhabitants of our geoid, leap seconds have the ignominious distinction of being the only part of timekeeping that is non-deterministic. DST is a pain, leap years are vaguely baroque; but leap seconds? Purely empirical. When team astronomy says that their little glowy lights aren't lining up with team atomic clock's buzzy little atoms, leap second.
Needless to say, this is absolute hell for any system trying to keep time with nothing but a good RTC and some tables. It's the single factor making it impossible, rather than merely subject to inaccuracy. TAI 4 lyfe!