Negative unread mails in Yahoo! %<:?

Today while I was trying to access my Yahoo webmail account, suprisingly after reading some mails, I met this strange behaviour from Yahoo Mail:


Then I suddenly recognized another unread mail and when I click on it, I came upon with this screen :This is a both scary and funny ; but probably occured because of a bug in yahoo. Oh my god -2 unread mail is this an Illusion? :D