The login page does double-duty. There is no dedicated registration link, so users must select "Login" then select "Create new account." In the reading order of the page, "Create new account" is near the end. On loading the page, the user name form control is focused.
The combination of these factors caused one user to mistake the login form for a registration form.
Why it matters
This was a very frustrating experience for the user. After entering a user name and password they submitted the form and the page reloaded. The password field was focused, and they missed the "Invalid login" message. They thought they had made a mistake with their user name or password, so kept trying different combinations, wondering if what they were entering was too long, too short, or contained forbidden characters.
After some time the user gave up in frustration, and had to be explicitly directed to the "Create new account" button.
Moodle should have a "Register" or "Signup" link preceding the "Login" link.