you mean spike? it coul be a fake spike registered in some exchange, i don't know from which range of exchange preev take his price calculation
I'am not sure what a spike means but Preev.com rates are absolutly and 100% legit as far as I know , using it for over a year now and he take from different sources : LocalBitcoins , Bitfinex , Bitstamp and also BTC-E
I was the only one who saw that price changing ?