On my Leafgreen game I accidentally skipped the Poison Gym for almost the whole pre-ending. I freaked, thought it would cause a sequence break(you know, where the game glitches and you have to delete your data, because you can't move forward story-wise?). Thankfully, nothing happened other than the Gym being easier than it would've been.
On the contrary, my Emerald game did get sequence-broken, unfortunately. I decided to LV up I bit before I tackled the Flying Gym, so I went to Lilycove City. My Rival challenged me to battle, and having him challenge me before I got the Gym Badge made it sequence break. (Darn Rivals.)
So why did the sequence break happen there, but not in Leafgreen? Is it because Team Aqua was in the city? Did it not have to do with my rival at all? Again, what determines the cause of a sequence break?