INSERT into companies (company_nbr,company_name,industry,status) VALUES (DEFAULT,'A new company name', 'Manufacturing',DEFAULT);
Yesterday, before learning to use DEFAULT for the company_nbr PK I entered all rows using company_nbr 2342-2391. This morning, after running the single-line INSERT command company numbers from 2341-2392 all had 'A new company name' as the company_name.
There is no way, in the absence of a user trigger, that the above insert command changed pre-existing rows. And if you cannot reproduce the behavior you claim to have seen I will continue to just assume you have faulty memory.