When 2 or more Authors are merged, the software doesn't change the "pseudonym" table. As a result, that table accumulates defunct pseudonym associations of various kinds. Depending on which Author ID is dropped, the exact symptoms may be different. We need to re-point all affected pseudonyms to the remaining Author ID when an Author merge occurs.
Anonymous
This also happens when authors are deleted when their last Title is deleted.
Fixed in mod/common.py 1.5 and scripts/delete_defunct_pseudonyms.sql 1.2. Implemented in r2009-54.