By default, Rails'
validates_uniqueness_of does not consider "username" and "USERNAME" to be a collision. If you use MySQL this will lead to issues, since string comparisons are case-insensitive in MySQL.
(If you use PostgreSQL, read this instead.)
Say you have a user model
class User < ActiveRecord::Base validates_uniqueness_of :name end
with a unique index in the database.
If you try to create the users "user" and "USER", this will not trigger a validation error, but may fail with an SQL error due to duplicate index key.
You can change Rails' behaviour, by saying
class User < ActiveRecord::Base validates_uniqueness_of :name, :case_sensitive => false end
When you get an
ActiveRecord::RecordNotUnique error (probably in combination with
Mysql2::Error: Duplicate entry) for a string field, case sensitivity may be your issue.