In most of the exchanges, a transaction must have 3-6 comfirmation before your transaction can be consider deposit. They don't release Bitcoin you deposit until that comfirmation is reach before they allow you to do anything with that Bitcoin, you may be allow to trade it with one comfirmation but you can't withdraw it unless that comfirmation is complete. Why?

Second question, is a transaction with 1 confirmation reversible? What are the chances of 1 confirmation Bitcoin reversible?