It looks like a trade of amount 0 has gone through and has caused the problem. This is the culprit:
{"date": 1311104701, "price": , "amount": 0}
I also noticed a trade in the order book earlier today where one of the quantities was zero so looks like the validation may need tightening up somewhere. (Edit: that's a guess, I haven't read the code in any detail)
That's a good guess.
The issue is that 2 trades went through in which one side got 0.
| 16947 | 18518 | 1 | 18539 | 0 | 2011-07-19 18:44:01 |
| 16973 | 18548 | 2 | 18562 | 0 | 2011-07-19 20:45:01 |
Those numbers are scaled 10^8, so those are trades for 0.00000001 for 0 and 0.00000002 for 0.