'-ible' suffix vs. '-able' suffix
I've always assumed that -able/-ible suffix patterns stem from the verbal conjugation of the Latin root, where 1st conjugation usually gives way to -able and the other three to -ible. Of course this goes for the Romance languages, not just English.
Disclaimer: this isn't a rule learned formally, more of an observation-based suspicion that never let me down through countless trials the SAT, my 7th grade spelling bee and that one semester when none of my courses were in English. Oh, Latin, where would I be without you? ;-)