Microfilm-readers and magnetic-tape readers of certain kinds. I don’t remember all the details, but back in the 70s, NASA sent a (rather expensive) probe into space and put the data gathered onto magnetic tape, which was the common storage media at the time. Two decades or so roll by during which they are too busy with other stuff, but then they want to look at the data again, and can’t read the tape because it has deterioated so much that it peels apart when put through the reader. (Okay, that’s strictly more under the heading of “limited longetivty of archive material” plus “failure to immediatly switch storage mediums when new tech comes out”*). So the whole scientific data, gathered at great cost and valuable to many science areas, is irretrievable lost. (Even sending a second probe won’t get the same data, as stars have shifted, gone nova etc.)
I think a similar case is with the US census taken in the 50s - it was stored on some computer-based medium which nobody has the reader hardware or software for anymore. The 1870 census, by comparison, was done on paper and therefore can still be used as baseline for comparisions.
I’ve forgotten which kind of magnetic tape/microfiche exactly, but there’s one kind which has only three machines in the whole world left, one in Germany, one in Japan, and one in the US(?). The companies that manufactured them went long out of business, the other institutions threw them away. If anything happens to one of the 3 remaining machines, the others will have to be cannabilzed for parts, because spare parts can’t be reproduced anylonger.
- This is also an unadmitted problem for libraries, esp. with limited budgets. In the 80s and early 90s computer programming books came with 5’’ floppies at the back, and were stored in the closed shelf. In the 90s, 5’’ are replaced by 3’’, and later by CDs. Theoretically, all major libraries should’ve gone through their catalogues and converted all floppies from big to small and then to CD. Practically, there was no budget to pay a full IT person to be aware of this, or to pay somebody to do the copying, and there was enough normal work to keep people busy, so the books with floppies are still sitting on the closed shelfs, only now no normal PC can read them any longer (if they haven’t deterioated already).