Ok, I know it's well known that you can light the torches in the Guilty Fire tier using Yunica EX version's Bolt Crash and therefore skip the first Kishgal fight and getting the fire skill. However, if you manage to get to the Devil's Corridor without doing this first, you can end up in an unwinnable situation.

This happened to me just now. Basically, skipping that section also skips getting the harmonica, which makes getting through the Devil's Corridor next to impossible. However, going back after triggering the Devil's Corridor event throws some characters' dialogue out of sequence. After the Kishgal fight, Yunica is teleported back to base and has to trigger dialogue in order to continue. And since the dialogue is out of sequence, the next event doesn't trigger and Yunica ends up stuck without being able to get back into the tower.