Was Nanoha critically injured during a Jail Scaglietti Investigation?
Before the events of StrikerS, although Jail Scaglietti's existence was a secret known only to the top tiers of the TSAB, his activities were already known, albeit the identity of its perpetrator remained unknown. In fact, said activities are one of the main points Hayate used during his exposition when forming Riot Force 6.

We can only deduce that Nanoha's incident happened in one encounter with Scaglietti's activities. Still, as there was no "Jail Scaglietti Investigation" back then, she must have been investigating a case of improper use of Lost Logia or usage of magical technology in an underdeveloped world (not specified, as far as I know)