Because the current price is just whatever a seller and buyer have 'agreed' to pay at any one time, if there are possible issues with bitcoins on one exchange (for instance, the real or imagined possibility that your bitcoins may go missing on MT Gox, or that there will be difficulties selling or retrieving the ones you hold in that exchange), people will be willing to pay less than the going rate for bitcoins on that exchange. Elsewhere, where this issue doesn't directly exist, people will still be willing to pay full price (though issues at MTGox may affect buyer confidence on other exchange, which leads to an overall decrease in Bitcoin price).
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Because the current price is just whatever a seller and buyer have 'agreed' to pay at any one time, if there are possible issues with bitcoins on one exchange (for instance, the real or imagined possibility that your bitcoins may go missing on MT Gox, or that there will be difficulties selling or retrieving the ones you hold in that exchange), people will be willing to pay less than the going rate for bitcoins on that exchange. Elsewhere, where this issue doesn't directly exist, people will still be willing to pay full price (though issues at MTGox may affect buyer confidence on other exchange, which leads to an overall decrease in Bitcoin price).