> The term “open source” software is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licenses that we consider too restrictive, and there are free software licenses they have not accepted. However, the differences in extension of the category are small: we know of only a few cases of source code that is open source but not free.
I was able to find one example, the NASA Open Source Agreement, which is accepted by the OSI [1] but rejected by the FSF [2]:
> The NASA Open Source Agreement, version 1.3, is not a free software license because it includes a provision requiring changes to be your “original creation”. Free software development depends on combining code from third parties, and the NASA license doesn't permit this.
[0] https://www.gnu.org/philosophy/categories.html
And possibly 9
> One important way to modify a program is by merging in available free subroutines and modules. If the program's license says that you cannot merge in a suitably licensed existing module—for instance, if it requires you to be the copyright holder of any code you add—then the license is too restrictive to qualify as free.
https://www.gnu.org/philosophy/free-sw.en.html#four-freedoms