Critical bug may be defined as a defect if not found will 
surely hampers your further testing for e.g if you click on 
your account number link and it does not move to the 
respective page of your online banking and there is no 
other way to get to that page, it is defined as critical bug

1.Banks provide a facility of paying to billers and this 
functionality breaks down .Site  does not allow you add 
billers ,delete billers or not validating adding a 
duplicate biller can be one of the critical defects.

2.We can tranfer money from one account to another 
account .Money account in which it is transferred can be 
from same bank or different bank .But the money from the 
account it is tranfered it has to be from the same bank.
Lets understand with example 
suppose we want to tranfer from ICICI to SBI bank and we 
have added SBI has one account in that profile .Then we can 
tranfer from ICICI to SBI not the SBI to ICICI via ICICI 
portal.

This is my real time experience..... I were involved in 
testing a banking application in UK. The interest accrual 
was happened on the next days BOD instead of previous days 
EOD.and the realization was on the next days EOD. This was 
the real critical bug I ve faced.

Hi This is chandra sekhar,
Here I am telling critical bug .
In data base table side there are two concurrency controls .
first one is optimistice concurrency control and second one 
is last one win concurrency control. optimistic concurrency 
control means a row in data base table can be updated only 
once. last one concurrnecy control means same row can be 
updated more than once. the requirement is optimistic 
concurrency control , but the developer developed with last 
one concurrency control. when I entered data in the 
textboxes with is primary key ID and submit the submit 
button, the data has been inserted into concerned table. 
Next time , When I entered different data and with same 
primary key ID this same rows has been updated. here is 
optimistic concurrency control voilated. 
Thsi is critical bug in my project.

In Banking: If money is transfered to another account than 
the one entered.

customer inserts the card in ATM to withdraw cash.
The amount is debited from the customers account but due to 
some reason customer didnt getthe cash in the ATM and the 
amount is not credited into the customers account back.
This is a valid and high priority bug.