This is almost certainly gits auto-merge files when there are no conflicts...
Developer A develops the 'protect your tweets' feature on master.
Simultaneously developer B implements the 'change email address' feature which involves creating a new user account and copying over all settings.
The two commits get merged, with no merge conflict (and therefore no manual review), but the 'copy over all settings' step now misses a setting and uses the default instead.
Developer A develops the 'protect your tweets' feature on master.
Simultaneously developer B implements the 'change email address' feature which involves creating a new user account and copying over all settings.
The two commits get merged, with no merge conflict (and therefore no manual review), but the 'copy over all settings' step now misses a setting and uses the default instead.