  Sharding, i.e. splitting the miners or validators to form and run several
subchains in parallel, is known as one of the main solutions to the scalability
problem of blockchains. The drawback is that as the number of miners expanding
each subchain becomes small, it becomes vulnerable to security attacks. To
solve this problem, a framework, named as \textit{Polyshard}, has been proposed
in which each validator verifies a coded combination of the blocks introduced
by different subchains, thus helping to protect the security of all subchains.
In this paper, we introduce an attack on Polyshard, called \textit{the
discrepancy} attack, which is the result of malicious nodes controlling a few
subchains and dispersing different blocks to different nodes. We show that this
attack undermines the security of Polyshard and is undetectable in its current
setting.
