I've been wondering about this for a while. It might even be impossible due to hash value of block being SHA256 value of SHA256 value of the block data, as far as I know (or is this only for Bitcoin addresses?) and there simply might not be a SHA256 value that produces 0 when it's hashed again, unless SHA256 outputs are very well distributed, which they might be in this case.
I have seen on some blockchain explorers that for the genesis block the previous reference block is 0. I thought that perhaps this would mean that Bitcoin blockchain might loop unexpectedly, but I figured out that coinbase transaction has 50BTC as an output, which would I guess make it invalid now that the block reward is bellow that.
Would there be any special problems with this? Obviously if the difficulty gets too high, block hashes will start overlapping which could create these loops, right? But I do understand that it is very likely that difficulty would have to grow quite unreasonably high for this, which it never might.