O.M.G! I FUCKING HATE THIS SO MUCH!
@ondrej I've heard the (security) reason this is done, is to avoid phishing. A legitimate website, knowing your username, can serve a personalized—familiar to the user—password input, with their avatar, name, or a secret third thing like my bank does. A mismatch should alert the user.
On the other hand, it's foolish to assume a malicious party can't easily scrap this from the original webpage.
@ondrej
I had an SMS authentication code sent to my phone that was like 14 digits recently.
Absolutely baffling design decisions, this stuff.
@ondrej It also causes me to type my password in the wrong field, readable for all around me, when the username or password is not accepted for some reason.