Error de SQL:ORA-00933:el comando SQL no finalizó correctamente

Punto y coma ; al final del comando me había causado el mismo error.

cmd.CommandText = "INSERT INTO U_USERS_TABLE (USERNAME, PASSWORD, FIRSTNAME, LASTNAME) VALUES ("
                + "'" + txtUsername.Text + "',"
                + "'" + txtPassword.Text + "',"
                + "'" + txtFirstname.Text + "',"
                + "'" + txtLastname.Text + "');"; <== Semicolon in "" is the cause.
                                                      Removing it will be fine.

Espero que ayude.