A miner has their clock wrong; there is no way to fake that previous block hash.
156325: d5e839cee timestamp 1323179658
156326: 9058e2de9 timestamp 1323179451
Nasty.
I suspect that long running nodes can have their time drift from the network, because the offset-from-local is only calculated once. If the local time then drifts, it doesn't matter that the network average would be able to fix it, because the offset is never looked at (I think; happy to be wrong).