I have the 20 Word pass phrase but blockchain is asking a 12 word pass phrase. I created wallet back in 2013-14.
After searching Google for a while I got this link
https://blockchain.info/wallet/forgot-password where I can enter my pass phrase and blockchain showed my identifier and password. The password and identifier are same as I know . But I'm still unable to login.
Yes that will definitely happen, because blockchain.info updated there wallet already to HD wallet with 12 phrases instead the 20 or 15 I guess I dont know the numbers . And what the hell you only remember this time that you have still balance on that shity wallet. Many users on blockchain.info go here with the same problem you had but I dont know if they solved there problem already.