(slightly different example of #19 )
https://osm.mapki.com/history/relation/84177
was deleted and then undeleted (i.e. changed from 0 parts to more than 0 via a "modify")
after that, and still at the time of writing:
gis=> select count(*) from planet_osm_line where osm_id = 84177;
count
-------
0
(1 row)
When the database is next reloaded I would expect it to reappear