You're right, it's unlikely anyone will accidentally do it. But I can guarantee there are malicious people around just itching to cause a bit of havoc in bitcoin. It would be best to plan for the case of "it will happen that someone will mine a block" rather than "I don't think anyone would waste time to".
Right........
If we're still at 55% on Feb 7'th then I'll be worried, too, and might advise miners to push the hard switchover date a couple of weeks (if they're using the patches I'm creating then it is a command-line argument to bitcoind).
Also, the worst case scenario isn't very scary-- worst case is the less-than-50%-of-miners who did the switch find themselves on the short end of a blockchain split, so they lose revenue (and transactions take longer to confirm because the network is working on two different chains).
That would motivate them to either quickly recruit more hashing power or quickly switch back to the old rules.