With instant_registration set to yse in the classic
interface:

Having clicked on "Register", the registration page is
opened; you fill in all your details and click
"Register". Naturally, you are instantly registered &
logged in. However, the page you are left at is the
registration page, with all your details shown; no
input boxes available, and a button marked "Register"
at the bottom. It looks to the uninitiated as if the
page is repeating your data back for confirmation, so
you click on the "Register" button once more, but get told:

"Error: Required user properties username, password,
address not supplied"

Of course in fact you are already logged in and don't
need to click the register button - but this is not
obvious.