Is it mandatory to provide details of all bank accounts?

Disclosure of all operative bank account details is to be necessarily shown in the ITR. If a particular account is dormant, then in that case, such information is not required to be shown. 

The consequences will be based on nature of transactions and incomes, which is not reflected in the return, which might be discovered at a later point in time.

Yes, it is mandatory, as elaborated by Satish.

Sir if the assessee has disclosed all the incomes pertaining to all the bank accounts & paid taxes due upon them but failed to disclose the account numbers. In such a case will he be penalised? 

You must have heard of Annual Information Return to be filed by various parties like Banks, Sub-registrar etc. This information is used intuitively by the department to identify such persons who have not made discloures or failed to disclose complete details to the department, where significant revenue is not paid to the Government. 

During the course of assessment, if the Assessing officer gets to know that you have failed to disclose certain income or concealed any income, then action will be initiated for the income escaping assessment.

If you have disclosed all the information and ensured the income tax is paid on the income earned then you need not worry. If the A.O is of the opinion that some income has escaped assessment, then he may call for such information and you can produce the same before him / her.