Strange. You should be able to check under "history" at what price your order was placed, and under "transactions" at what price it was executed. I guess ideally you would also need a way to look up the order book at the time of execution to prove that something went wrong.
Yes, it isn't easy to prove but I think the balance of probabilities is one-sided. Transactions show
Buy April 25, 2013, 3:02 p.m. 1.00000000 BTC -$170.28 $170.28 $0.65
History shows
April 25, 2013, 3:02 p.m. 124.171.69.22 Opened limit buy order (amount: 1 BTC, price: $170.28)
So the order was filled at the limit, which was set at 10% about the last traded price when it was entered. This was attempting to simulate a "market" order. Whatever the limit an order should be filled at the best available price.
Oddly
bitcoinity doesn't show any trade over $161 for that period. It shows up in the trade data from
bitcoincharts though:
1366902132,153.000000000000,0.852534330000
1366902132,152.100000000000,0.147465670000
1366902127,170.280000000000,1.000000000000 ***
1366902124,152.100000000000,0.500000000000
1366902124,152.100000000000,0.500000000000
1366902124,152.900000000000,0.827534330000
Maybe the heavens parted in those 3 seconds between 1366902124 and 1366902127 and dozens of traders all pulled the asks. It doesn't seem likely, though.