Usually the “[sic]” goes after the thing you’re pointing out.
Whoever thought that was a mistake probably thought the word needed to be “incidental” to be used as an adjective. Perfectly ok in Madison’s time, though.
Usually the “[sic]” goes after the thing you’re pointing out.
Today it would likely be written with a comma, though it's not necessary. Unlike Madison, the author probably thought incident had to be a noun.
in·ci·dent -
noun
1. an individual occurrence or event.
... (6 more)
adjective
7. likely or apt to happen (usually followed by to ).
Thus, "It is a misfortune [which is apt to happen](incident) to republican government."