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phamilton touched on another good example - the budget can be implicitly set per-user via a frequency cap. If you see the user and have a chance to bid on an impression for them, the odds are good you'll see them again in two seconds -- winning two auctions means you've overspent by 100%. Oops.

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Perhaps also worth pointing out that in RTB, quite often you won't know if you've actually won the auction or not for a few seconds later (when the exchange calls you back with a win notification, or you get it directly via a pingback from the user's device)

During that delay you might have actually already processed new bid requests (auctions) for the same user.

Depending on the order's characteristics and how much you're willing to deviate from target - especially when observed within a small time window - the above poses additional challenges w.r.t. overspending.

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