The two threes touched at the corners, and I missed it
The grid was almost done. I stared at the last open region while the mistake sat in the opposite corner.
Small regions closed first: where only a two-cell group fits, there is nothing to decide. Then the threes fell into place, then the bigger numbers. One middle region stayed open. I tried layouts, erased, tried again. The group kept breaking apart or drifting from its number. No 2x2 squares anywhere. Everything checked out except the count.
The middle looked guilty
I rechecked the middle twice. Connected, right size. So the fault was elsewhere. I walked the solved regions one by one and recounted them. Two threes in different regions, each neatly connected, touching at exactly one diagonal corner across the border.
My eyes kept reading them as separate because the numbers lived in separate regions. The rule disagrees: same numbers never touch, not even at corners, and region borders change nothing.
What I check first now
I erased one three, rebuilt both regions, and the grid closed in three minutes. The "Hint" button could have opened one correct cell and pointed at the spot all along. I never pressed it: I was sure the middle was guilty.
Since then my habit is fixed: when a solve stalls, I check same-number touches across borders before touching the region where I got stuck.