Assume a user got their username or password wrong. What is more likely, they got their username wrong in a way that happens to be the same as another user, or they got their password wrong?

Most people store passwords in their browser, which already makes the answer not that obvious.

For a rarely used web site, I honestly would have no idea if I registered as bonzini, pbonzini or bonzinip. Now my surname isn't particularly common, but smithj and jsmith might be easily confused.

I have a commonish firstlast@gmail account and frequently receive what is the obvious result of people in various parts of the country incorrectly providing my email to apply for jobs, order pizzas, and use other services.

It is absolutely common for users to supply the incorrect username/email.

This is why it is common practice to require the user to enter their email address twice in forms.

Thank heavens we never invented copy and paste, would certainly make this more of an issue.

Apparently the set of users who don't know how to type their own email and the set who know how to use copy/paste are non-overlapping.