I have player "slitherlink" (or "Loopy" as it is sometimes called) generally on a 20x25 grid, so I don't finish too quickly.
However I have come across a puzzle (seed was not recorded), that MUST be solvable, but every attempt (2 dozen or so times) results in a impossibility.
Attached is a photo of the corner of the puzzle. The inconsistency happens while solving this corner, there has been no need to leave the bounds marked, before it fails. There are 2 zero squares, and a couple of 3-3 3-2-3 patterns to start from.
This is failure is driving me crazy, and I have been very very cautious in checking each of my moves.