Too many transactions are allowed for amount coming within user balance, but the entire transaction value is above user balance. For example, In Unwrapping, I had executed a unwrap for 3k sUSDT, another one for 5k sUSDT and then again 1 transaction for 5k sUSDT. My actual balance was only 5k sUSDT. So system allowed me to do 3 transactions as the balance available was never updated after 1st transaction or second transaction.
Ideally, after first transaction the amount involved in the transaction should be locked. only remaining amount should be made available for next transactions. Otherwise every Blocks will end up having many invalid transactions because user balances displayed are not updated evefrytime they do transactions.