This can be something that is extremely frustrating for anybody. When you spend a lot of time on a quest, you get satisfaction only when you can see the quest near completion, and when you find yourself stuck because of some issue, it can feel real bad. Something similar happened to this person (a senior citizen). He was given a large jigsaw puzzle (with around 5000 pieces), a puzzle over 5 feet long, by his daughter-in-law many years back as a Christmas present, but is no longer supported by the original manufacturer of the puzzle.
Now, in order to ensure that his puzzle gets completed, he is looking for the missing piece of the puzzle; but he is out of luck in getting the piece and the missing piece cannot be found anywhere in his house (one could think that they have completed the puzzle with every other piece inserted but the final piece, but it is not the same) (link to article):
The process was nearly complete when he realised that a vital piece was missing. The jigsaw showed the 19th century oil painting The Return Of The Prodigal Son by James Tissot.
arris searched his home in Shepton Mallet, Somerset, for the key piece, but to no avail. His family now fears one of their two dogs may have swallowed it, Daily Mail reported.
Help was sought from the puzzle's manufacturer to provide a spare piece, but Harris had taken so long to complete it that Falcon Games has stopped making it.