Note that the problem might arise also by a bug / accident and not as an attack.

Since value spent is not part of the signature it is easy to create an 
arbitrary fee by a defective wallet software.
Collecting that huge fee might provide a higher incentive to miner than the 
block subsidy on the trunk.

Assuming miner are fully rational, they might even form a temporary coalition 
to claim the fee:
The miner who mines forking block might offer part of the fee gained in a 
similar transaction to
other miners, so they help to extend his fork. A sufficiently high stake could 
trigger a long
fork “battle” of ad-hoc coalitions.

Addressing the known bug of the signature hash, that it does not include the 
value spent,
would have other positive effects, e.g. for resource limited hardware wallets.

Interpretation of an OP_NOP for a value hashing signature check were suggested 
by Alan Reiner
discussed earlier on bitcointalk.

Tamas Blummer
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