Oracle start database with incorrect password
When your OS user is from a DBA group, you can connect AS SYSDBA with OS authentication:


  Two special operating system groups control database administrator
  connections when using operating system authentication. These groups
  are generically referred to as OSDBA and OSOPER. The groups are
  created and assigned specific names as part of the database
  installation process. [..]
  
  Membership in the OSDBA or OSOPER group affects your connection to the
  database in the following ways:
  
  
  If you are a member of the OSDBA group and you specify AS SYSDBA when
  you connect to the database, then you connect to the database with the
  SYSDBA system privilege.
  If you are a member of the OSOPER group and you specify AS SYSOPER
  when you connect to the database, then you connect to the database
  with the SYSOPER system privilege.
  


You can then use CONNECT / AS SYSDBA or even CONNECT FOO/ANYTHING AS SYSDBA, the DB will ignore the user/password.