Same query, different result (depending on user's permissions)
If two users execute the same sql statement, is it possible that user1 gets different results than user2? 


Yes, absolutely. 

Row-Level Access Control allows the Database Administrator to put "contextual" filters on a table so that different users really do see different data.  

For example, a Users table might be protected such that users in the Personnel department can see everyone but anyone in any other department can only see people in the same department.