Hi Ganesh, this could indicate "data corruption". If it would try to find information via an association, this would simply return empty values. Now it is looking for a specific entry in the database.
If you have relations that are referenced in both entities (ie many-many relations), be aware what happens if you delete a record . This does not automatically update the related record. On delete, you should first remove the object from the relation and commit the related record.
It's weird that this question does not come up more often (I think this bug is fixed in later Mendix versions, maybe that explains it).
You're running into a Mendix bug. When you have two records that are associated with each other, and you delete one of them, Mendix forgets to delete the record in the table that links the two records. So the remaining record thinks it still has an association, when Mendix tries to retrieve over this association, it is unable to retrieve the other record (obv, since you deleted it perviously).
As a workaround you would have to manually unset associations to the object you're deleting when deleting it.