I tend not to blog too much on subject lines as they are really a marketing issue and a subscriber relationship issue. The subject lines a particular mailer uses should be directed and developed with an eye towards making the mail relevant and useful to the recipient.
What subject lines shouldn’t be is deceptive, either intentionally or inadvertantly. How can a subject line be inadvertantly deceptive? Take this: “Today only! One day sale!” The email in question was a printable coupon to get a discount at a bookstore. Unfortunately, the sales was not “Today” – the day the email was received.
On the one hand, I can sympathize with the sender. Sometimes email takes a while to get delivered, particularly for large mail drops. So you want to send before the mail needs to be in the inbox and in front of the recipient. But, that means that some of your recipients may get the email before “Today.” A much better subject line would have been “Friday only! One day sale!”