is there any way to record program flow for error checking later?

Our Clients sometimes complain about program not working in the desired way. We spent hours testing and sometimes are not able to find how that chance for error
occurred. If there was any way to record the program flow it would have been easy. Its sometimes because there was an attempt to convert varchar to int. How can we achieve this?

example:
created variable x
assigned 23 to x
passed x to sub root()