Consider the following scenario:
January 2016 block A gets submitted. Its hash is
January 2017 block B gets submitted. Its hash is also
10 minutes later, block C gets submitted and the previous field points to the hash
Is it still possible to determine that the previous block is block B and not block A?
Note that while this is unlikely, there are not 2^256 possibilities a block hash can be. Every block hash has to start with zeroes which decreases the amount of possibilities. In this scenario there are only 2^4 possibilities even though the hash has 7 bits which would theoretically allow 2^7 possibilities.