So, I'm currently doing the main quest, yeah? I just completed "Offerings for the Shrine"
See here:
https://gyazo.com/379072ad1d56cf36b5619cce03a2800f
I figure, great! Doing 3 dungeons in a row was pretty strenuous, can't wait to move on!
Except...
https://gyazo.com/7a530d9d0c56b7826b7690f3be02c044
Oh, guess my inventory's full. That's odd, I don't recall ever having a full inventory space... Lemme just check and see--
https://gyazo.com/4a9b20014c5edb04cd09e8b311d1199b
Oh, what's that? only 21/100 slots are being used and my inventory ISN'T in fact full at all?
Isn't that a neat trick.
So guys...
What do? I can't continue the game because it returns a "Can't complete quest, inventory is full" message every time I try and turn it in.

PkFireAlpha wrote:rant

okay so I'm an asshole
I had too much of the food item on me to complete the quest.
I sold some to make room for the new stuff and now we're good.
rip me haha

This is actually really, really common. I guess I'll add it to the FAQ.