When Criterion restores movies, the company prefers to work from the best possible source material—typically, the original negative. But in the case of Alfred Hitchcock's 1934 film The Man Who Knew Too Much, the original negative was missing. Looking for the next best option led Criterion's restorers to a 35mm nitrate print, but it had lots of scratches, so they opted to hold out for something better.
Ten years later, they finally found what they were looking for, but even then the restoration wasn't easy: The process involved consulting a film shrinkage expert, finding a specialized scanning machine, and much, much more, which you can see in the video below.
The fully restored The Man Who Knew Too Much is currently available on DVD and Blu-ray.