In a recent article on FSF.org
** The main license in actual use that isn't reasonably behaved is the license of TeX: if two programs are licensed just the way TeX is, there is no authorized way to distribute a merged version of them.
The TeX license permits distribution of a modified version only in the form of the original version plus a differences file. If A and B are separately released that way, then merged, distributing the merged program as A plus a change file violates the license of B. Distributing this as B plus a change file violates the license of A. Distributing this in any other way violates both licenses.
It is no coincidence that TeX was released in 1982: our community has learned, since then, to write reasonably behaved licenses.
Do modern TeX distribution suffer from this issue, or is that just ancient history these days?