Half of my PhD thesis is considered "unpublishable" because, after doing the work, my supervisors felt it's actually "unsurprising" that it didn't work out. We took methods that had been exploited to improve on previous results for over a decade to their logical extreme, and found that this method no longer leads to improvements. After doing the work it seems obvious. A paper on the subject would almost be considered uninteresting, and a high ranking journal would ignore it (which is why it's considered "unpublishable"). However, nobody has published this information, and it would help others to not make the same mistake.
I wonder how many times similar "mistakes" have been made by PhD students across disciplines.
I wonder how many times similar "mistakes" have been made by PhD students across disciplines.