What exactly is the reason for problem if replication factor is more than the number of nodes

[Monica Shiralkar]

I read that if say the replication factor is 3 (default ) and the number of nodes is 2 then it creates a problem and hadoop expects the number of nodes to be atleast equal to the number of nodes in the cluster.But what exactly is the reason for this problem ?. Replication factor means the number of times each block is replicated on the nodes.
Thanks