ffmpeg's reverse-engineered Real Video decoder is an open-source implementation of a proprietary specification. Depending on the country and legal interpretation, the mere existence of this is illegal.
ffmpeg's H.264 decoder is an open-source implementation of a patented, but not proprietary, specification.
ffmpeg's Theora decoder is an open-source implementation of a probably-not-patented specification.
If a definition doesn't distinguish between something freely available for all to view/implement and something which cannot even be implemented without potentially falling afoul of copyright laws and the DMCA, it isn't a very good definition.