So, there's something really weird in the text of contingency. Contingency says that
"You can use only one contingency spell at a time; if a second is cast, the first one (if still active) is dispelled."
Now, I'm not sure if what I'm about to propose works. The reason I'm not sure is that "useing" a spell is not a normal term in the game rules. However, in my view, the natural way to take this is that the second clause explains the first. The reason you can only use one contingency at a time is that whenever you cast a new one, the old one is dispelled.
Exhibit b) "Spell Rebirth." Lexicon of the Evolving Mind 4. This restores one effect that was "dispelled" in the previous round.
So, we cast a contingent fireball (for some reason.) Then we cast another contingent fireball. Once the second one finishes casting, the first is dispelled. We immediately use Spell Rebirth. The old contingency, which was, after all, "dispelled," is immediately "restored" and "resumed." It is not, however, "cast." This means that the dispelling clause does not kick in, and both the restored contingency and the second cast contingency are now in effect.
Getting more than two requires either a) multiple spell rebirths or b) taking literally the clause that says that if a second contingency is cast the first is dispelled. If this is all that is true, then casting a third contingency will not dispel anything. Even if it dispels only the previous one, one spell rebirth will allow us to continue the contingency chain.
(At epic levels, "Tenacious Magic (contingency)" would do this far more effectively)
What say (well, I mean, this is pretty clearly stupid and few DMs would allow it, but is it technically RAW? Or have I already stretched RAW in my attempt to figure out what the 1-contingency clause actually means in game terms?)