There's actually a few of them, but the most recent I can remember off the top of my head is probably the DRM leasing. That was a fun one. Everyone got together discussed how to do DRM leasing, gnome agreed and signed off on the implementation, stayed quiet for a long time, someone made a pull request for the agreed upon implementation, silence for a bit, and then all of a sudden "actually this implementation bad should be portal so nvm not doing it this way everyone else should change to use portals"

ah right another was variable refresh https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1154 basically sat for 3 years with no review, and when users started being like hey we really need this what's going on developers got all super defensive and ultimately locked it claiming harassment

The comment about 8 years was a reference to the thumbnails in the file browser. If I recall correctly that one took about 8 years