If the database of a bookmarked game is sorted or compacted, the bookmark details may become out of date. When that happens, Scid will search the database for the best matching game (comparing player names, site, etc) when the bookmark is selected, so the bookmarked game should still be loaded. However, if details of the bookmarked game change, it is possible that a different game will match the bookmark details better and be loaded instead. So it is a good idea to re-bookmark a game if you edit its players, site, result, round or year.
TODO. Add this as a question. Put a link both here and on Compact Databases