Not exactly a security bug, but there was a company that made a Django app for some internal work and later open sourced it. I was browsing through the code and I saw that the config file had an IP address and a hashed password for the database credentials

When I tried to use them, I was able to login directly to their read replica RDBMS, I had access to all their customer data (including phones & home addresses)
Being the saint I am, I informed them of the ignorance made by their developer and was presented with some cool swag.