Oracle 11g user created with Create session but able to update the table? I created a readonly user with only permission granted was create session and read only privilege, using the link:
https://stackoverflow.com/questions/7502438/oracle-how-to-create-a-readonly-user

Now when I login with the new user I created I am able to update the table. 
Can anyone suggest me what I could have done wrong?
There is no such thing as a read only privilege. If someone grants update,delete,insert on x to public, everyone who can create a session can modify the contents of that table.

Also, if you create a user x and using a dba account create a table x.y, the user x - who owns the table y - can modify the contents of his/her table, because he/she is the owner of that table.