All of a sudden, I received 50+ undeliverable mails in my Yahoo! mail inbox. I realized someone just sent spam mails to all my contacts in Yahoo account (150+) using my account. Luckily, most of the contacts are not up-to-date, and the mails sent were bounced back to me. Because of this, I was alerted and changed my password in consequence. I feel sorry for my friends.
A quick search from Google turned out that it is quite normal to have your Yahoo! mail account compromised: there is quite a few incidents happening quite recently. I remember that I also received spam mails from my friends sent from their Yahoo account “from time to time”.
Update: 18 June 2013
I still get some failed delivery emails today at exactly the same time as yesterday (However, no more suspicious login to my Yahoo! mail account). Seems like the spammer has set up a scheduler to send the spam mails with the stolen contacts.