That's exactly what happened to me on my driver's license this year. I got the renewal in the mail, and thought "I'll do it later" on-line. A week or so AFTER my birthday, I was driving to work, and realized I was driving on an expired license. Luckily, I was off a couple of days later, and was able to catch the DMV open.
Then I checked on renewing the permit so the same thing wouldn't happen again. I thought about doing the lifetime since renewing was $50.00 every 4 years, it would only take 16 years to pay for itself. Then once I realized it was only $50.00 for EIGHT years of renewal, lifetime didn't make a lot of sense for me.
I renewed in February, and mine didn't expire till August, and like it was said, my first thought was that I cheated myself out of 6 months, but 8 years from now, who cares. I look at it as getting 42 extra months for the same price that I paid on the last renewal.
I also received the renewal in 8 days or so.