Thanks, I'll put that change in. Do you recall what action you had just completed when the cursor locked?
Since it happened while I was playing near the top, I had a feeling it was similar to one I had that happened as a result of having selected and trying to match a fruit as it was getting eliminated as waste. Oh, and it doesn't get locked, but rather displaced by one on Y (you can keep playing "normally", except you select the tile above the cursor).
The only way I managed to reproduce it, was by leaving a lot of single tiles, selecting them and trying to match them as they were getting eliminated. It's hard to pinpoint the exact trigger conditions because you only realized it happened afterwards, when you're trying to select another tile.
It could be unrelated, but when it happened, a weird glyph appeared on the last digit of the score (went back to normal as I kept playing).
Thanks very much for the testing, I've re-written how the cursor sprite is placed, before it was being moved manually whenever an action happened (I was being lazy...) so it was possible for its screen position to get out sync with the selected cell. I now have a lookup table to re-position the cursor every frame based on the row/column of the selected cell.
I think the score glitch is because the score uses binary-coded decimals, when any component of the score calculation is more than 9 it's putting the total out of BCD range. Should be an easy fix.