When you forget to bring your own lock picker character...
So I found out it hard way that you can't finish chapter if you use one of the keys found on this level at center treasury room. In fact I was forced to play this ch 2,5 times. At first I got careless and Harken killed Lyn. Second time I played it fully trough and realized at the very end that I can't open all doors. Ant to add to the insult this ch sports snowing which slows your movement quite a bit.
I wonder if it's the only FE game where you can block completely your progress? So far I have finished Path of Radiance, Awakening, Fates, Shadows of Valentia without facing this kind of problems. Maybe I have been just lucky.