Chapter 21 pulled such a jerk move at the end.
I had everyone near the boss with a load of the big stone guys. The only ones who could reliably fight them were Ryoma and Rinkah so I used them to tank and just kept everyone else out of range.
Then suddenly the place gets covered in lava.
I saw the little "Heart -10" which I thought meant it would take 10 HP every turn from anyone standing on it, so I bum rushed in with everyone else just so I wouldn't have to worry about Rinkah and Ryoma getting done in by the lava.
But it only takes 2 HP.
And then Lucina got screwed by RNG and got hit by to hits in a row that had like a 30% Hit Rate. If she got a critical in she would have been fine too.
REEEEEEEEEEESET.
Oh, and Rinkah only got a handful of EXP from the boss. It's actually really hard to Level Up with her now unless it's a character boss (like one of the Nohrian royals or something).
At least I got this cool screenshot though.