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Now let's consider a bit more complex case: suppose that packet which was carrying request body was buffered at some router between user's computer and MPex server and was "in flight" for some considerable time, e.g. 1 minute. It's possible in case of congestion, packet loss/retransmission, routing problems etc.

In that case we can a situation like this: 1) user clicks "stop" button in browser, but it doesn't stop packets which are already in flight; 2) user checks state, it says that order is not accepted; 3) now those packets which are in flight arrive to server and it actually accepts the offer. Whoopsie.
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