The assumptions I most frequently run into are that DBAs are a "catch all" for any database issue, no matter how remote.
If the word database is used in the description of an issue, then it is a DBA issue. If the name of the company that released a DBMS is used in the description of an issue, then it is a DBA issue, even though that company builds/sells other software.
I also run into the assumption that the DBAs know everything about every application and the way each application uses a database.
When I support a new (to me) database, I have to let go of my assumptions that best practices have been used. Sometimes, valid reasons (political, financial) cause the DBAs to overlook best practices.