And here I was clicking on this node thinking that I was going to find a warning not to attempt to differentiate a function which is continuous but is not smooth in its first derivative, eg: f(x) = abs(x*ln(x^{2})) in a neighborhood around x=0 looks like:
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Which many calculus textbooks call a function with a "sharp point". The limit of f'(x) from the left is minus infinity, and from the right is plus infinity, so from an analysis standpoint this is the sharpest point you can have. Be careful!
I leave it to Hallmark as an exercise to come up with other math warnings: CAUTION: NONREMOVABLE DISCONTINUITY, etc.