Let us call any expressed case of unexpected behavior a complain. The term is vague enough not to imply that it is always a problem or an error, which was introduced during development and needs to be fixed. Working on a complain starts with investigation. The only thing that you know from the beginning is that something on your website does not work for someone. The incidents raised as a result of automatic monitoring systems are a separate case.
Surprisingly, people in general are rarely capable of answering the question of "What exactly does not work for you?" Some of them get confused or even angry. While you must always ask this, do not get high hopes. Let us analyze some of the possible answers from the most popular to less so:
"Nothing!"
"I load the page, I click there and there, order a book, get to my shopping cart, initiate payment, get confirmation and on the last step there a security warning about some sort of certificate which expired yesterday"
Most of...