By chance, I found a closed natural=coastline way inside another closed natural=coastline way. It had been in the database for 9 years.
Curiously, the coastline tool of OSMI doesn’t detect this as a coastline issue, but this could obviously cause problems for people trying to consume OSM coastline data.
If you have a planet file loaded up, and like writing queries, making sure there aren’t any more of these would be helpful!