The belief that inserting a leap second is an ordinary tick that software need not model. Drafted here as a falsehood to bind evidence to; nothing is established yet and no source has been reviewed for it.
Treat a clock that can step, repeat or stall as a hostile input, and never let a wait be bounded by a wall clock you do not control.
Editorial judgements on a 0 to 10 scale, argued in the notes below and nowhere else. They order a list. They are not evidence and they never appear in a claim.
Draft. The candidate that would bind to it is the 2012 leap second, whose primary evidence, an operator post-mortem and a kernel commit, probably still exists. Scores are a first guess and carry no argument yet: damage is pencilled high because the reported failures were simultaneous and widespread, which is exactly the claim that still needs checking rather than assuming.