When an object relationship is defined to a table, (let's say articles has the relationship author to authors table), un-tracking the remote table (author) results in an 500 (with a message along the lines of 'could not find table "author"').

This happens because the object relationship author on articles does not assume a direct dependency of author table (which is incorrect), so author relationship isn't dropped and when adding this to schema cache, we get a 500 as we can't find the information about author table as it is untracked.