Corresponding source
The GPL's requirement for exactly which source code must accompany a binary – and the part of the license most often broken.
Posting a tarball of "roughly the right" version does not cut it. GPLv2 already demanded the complete source code plus the scripts that control compilation and installation, so the recipient can actually build the same program. GPLv3 introduced the term Corresponding Source and went further: for consumer products, keys and installation information must also be handed over if the device only runs signed code. That was the answer to TiVo, which followed the letter of the license and locked down the hardware anyway.
The source can ship with the product or via a written offer that, under GPLv2, stays valid for at least three years. Routers and smart TVs with a dead link in the manual are still routine. Such cases make up most of the compliance work Software Freedom Conservancy pursues.