This usually happens when you restore a database. For example, you restore a copy of production database X to your QA server. In essence, you have overwritten the user info of that specific database with what exists in production. This creates an orphan user -- where you have no login associated with a user in that database on a server that once associated that user with the old copy of the database.
Here is what to do fix the issue:
1. Validate that what you think is the problem is the problem. Do this by listing the known orphans. You may have others show up--but be sure the id you are interested in is on the list:
use [your database instance]
go
EXEC sp_change_users_login 'Report'
2. Fix the broken login:
EXEC sp_change_users_login 'Auto_Fix', 'orphan username'