"Or later" clause
The wording that lets users pick a later version of a license than the one the code shipped under, handing part of the future to whoever writes that license.
The GPL's recommended header says the program may be used under version 2 of the License "or (at your option) any later version". Writing that means trusting the Free Software Foundation not to produce a future version you dislike. Linus Torvalds did not. The Linux kernel is explicitly GPLv2 without the clause, so when GPLv3 arrived in 2007 the kernel could not have moved even if it had wanted to, which it didn't.
For years the distinction was invisible in metadata, which just said "GPL-2.0". In 2017 SPDX deprecated that identifier in favour of the unambiguous GPL-2.0-only and GPL-2.0-or-later. GPLv3 offers a middle path: the author can name a proxy who decides whether future versions are accepted.