+1 for vector76's hypothesis from me as well.
This seems a lot more likely than my double block mining hypothesis. This would also be harder to protect against.
Anyone have ideas about how to stop this sort of attack?
When the block chain forks, the "true" chain is ambiguous by design. The algorithm only promises that it will resolve eventually.
Wait, I thought this statement isn't true. My understanding is that potential forks in the chain are resolved according to whichever winning hash has the lowest absolute value (highest difficulty). This scheme is at least deterministic.
Is this correct?