Using GetObjectsMatchingObjectSet, GetObjectsRelatedToObjectSet, and GetObjectsRelatedToObject on IRelationshipClass return the following error if the object has more than 1000 related objects:
Underlying DBMS error[ORA-01795: maximum number of expressions in a list is 1000]
This only occurs when the underlying database is Oracle.
Oracle limits the number of items in an 'in clause' to 1000 items. ArcGIS doesn't break up the 'in clause' into multiple queries when retrieving all the related objects.