Square added a patch to chapter 13 which lets you follow one of two storylines. When Noctis gets separated from Gladio and Ignis, you can either go through chapter 13 as Noctis for a few hours or follow the alternative path of Gladio for roughly a half hour.

I thought this was a throwback to FF6 ala Kefka's Tower where you can switch between the parties to progress, so I chose the Gladio path and then realized you can't switch paths. Oops.

Was just curious if anyone else did this, too?

I only found out after the fact that if you go through the Noctis path you can still access the Gladio path via the title menu later, but choosing the Gladio path makes you skip the Noctis path entirely until they converge again.