Addendum to glitch: it appears that the previous glitch can cause one item slot in your inventory to get doubled up. The two slots remain linked (selling an object from one decrements both), so there's no benefit, but it takes up two against your limit. At least selling all from that slot fixes it.
Have a kind of equipment forged to two different pluses in you inventory. Use the museum tab on the right-hand side to forge & donate something. It'll charge you for two copies of that thing.
1: Clicking "Back" from a puzzle should take you to the puzzle select screen for that difficulty, not all the way to the top.
2: If you haven't made any moves, the "Back" button shouldn't trigger a prompt.
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