After you executed the migration using rake db:migrate and found out that some of the migration script is not executed ex. 20140515151637_add_missingfield_on_user_table.rb. But if you’re going to look at in the schema_migration table it was already saved.
So that means that 20140515151637_add_missingfield_on_user_table.rb file was already executed but it doesn’t update the affected user table.
Table schema_migrations:

| 20140514054402 |
| 20140514054742 |
| 20140515151637 |
| 20140526094004 |
To fix the problem, you need to delete first the record(20140515151637) from schema_migration table and then execute the specific migration version:
rake db:migrate:up VERSION=20140515151637