Prevent Keychain from copying an internet password to my login keychain
This was a security bug and a fix was released today by Apple.

About the security content of Safari 10.1:

Safari Login AutoFill


  Available for:  OS X Yosemite v10.10.5, OS X El Capitan v10.11.6,
  and macOS Sierra 10.12.4
  
  Impact: A local user may be able to access locked keychain items
  
  Description: A keychain handling issue was addressed through improved keychain item management.
  
  CVE-2017-2385: Simon Woodside of MedStack