Video transcript
Can every nonogram be solved without guessing? No—not every possible clue set. A carefully made puzzle is usually designed for logic, but the clues themselves do not guarantee uniqueness.
Consider two five-by-five boards where every row clue and every column clue is one. A diagonal grid is valid, and the reverse diagonal is also valid. Every line contains exactly one filled square, so the same clues permit different answers.
That counterexample proves three ideas must be separated. A grid is valid if it obeys every clue. A solution is unique if only one finished grid works. A next move is logically deducible only if it can be proved from the current information.
On the ambiguous example, a corner is filled in one valid answer and empty in the other. Choosing it because it looks promising would be a guess. A safe move must survive every remaining valid possibility.
When stuck, recheck full lines, overlaps, completed runs, separators, and crossings. If those prove nothing, the puzzle may need a deeper deduction or its clues may allow multiple answers. Possible is not the same as guaranteed.