Why I get "Please reenter the verification code" message when login or register?

Please explain what happened.

A user get this message in two cases: * The number on the right side of the last input box is the typed in correctly when login. * The page was loaded in a while ago so that the effect of number has expired. In both cases, one can click the said number to get a new one and then type it in correctly inside the corresponding input box.

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