Introduction:
In the cash book, a discount allowed of 410 has been entered by mistake in the bank column. This error needs to be rectified in order to maintain accurate and reliable financial records. Rectifying the error requires the identification of the correct entry and the appropriate adjustment to be made in the cash book.

Identification of the error:
The error can be identified by comparing the cash book entry with other supporting documents such as invoices, receipts, and bank statements. The incorrect entry of discount allowed in the bank column suggests that the transaction might have been incorrectly recorded as a bank transaction instead of a cash transaction.

Rectification of the error:
To rectify the error, the following steps need to be taken:

1. Identify the correct entry:
- Review the supporting documents to determine the correct entry for the discount allowed.
- Verify if the discount was given in cash or through a bank transaction.

2. Determine the impact on the cash book:
- If the discount was given in cash, the entry should be recorded in the cash column of the cash book.
- If the discount was given through a bank transaction, the entry should be recorded in the bank column of the cash book.

3. Adjust the cash book:
- If the discount was given in cash, a new entry should be made in the cash column to reflect the correct amount of the discount allowed.
- If the discount was given through a bank transaction, the incorrect entry should be crossed out in the bank column and a new entry should be made in the cash column.

4. Reconcile the bank statement:
- After making the necessary adjustments in the cash book, the bank statement should be reconciled to ensure that the correct balance is reflected.

Conclusion:
Entering a discount allowed of 410 in the bank column of the cash book by mistake can lead to inaccuracies in the financial records. To rectify this error, it is important to identify the correct entry and make the appropriate adjustments in the cash book. By following the steps outlined above, the error can be rectified and the financial records can be maintained accurately.