Did you buy the new-and-improved 2015 Chromecast? If so, you may have forgotten that your purchase includes a free movie rental on Google Play Movies, even if you'd already purchased a Chromecast previously (offers are per device, not account - so you get one for each CC you purchased up to a maximum of five). The movie rental offer doesn't apply to Chromecast Audio purchases, in case you were wondering.
The current offer runs until the end of the year (12/31/15), so there's no rush to redeem it. As always, in order to redeem the offer, you'll need to open the Chromecast app on your device or head to cast.google.com/chromecast/offers while on the same network as or nearby your Chromecast device.