When I say relatively easy, I mean compared to the prevailing wisdom that causing a fork requires controlling 51% of the total bitcoin mining capacity.

Also, it doesn't need to be a crypto failure. Just a divergence of the clients on how they handle the protocol. Possibly a discrepancy that would be very unlikely to occur by chance or in testing, and only manifests itself when a malicious user triggers it. This is the threat model bitcoin has to worry about now...