I may be a few days late on this but it's been suggested (not yet sure proven) that you are guaranteed to get a legendary on your 40th pack if you did not receive any in the last 39. All 40 packs must be of the same set (so 40 Classic not 20 Classic and 20 GvG).
Of course I go and buy 40 packs cuz what the hell. I bought GvG packs since it was announced that you can no longer buy them after the next big expansion/adventure hits. Mind you I don't know when the last time was that I opened a GvG pack and got a legendary but, 11 packs in I get a Golden Blingtron. 4 packs later I get Bolvar. I think it was 12 packs later and I got Mogor the Ogre. I'd say I got really lucky but if anyone else has attempted this let us know how many it took until you got your Legendary. Maybe we can debunk this theory if it wasn't officially proven by Blizzard.
And BTW I'm really not trying to brag.