It is possible for there to be some overspend in the case of a very small campaign budget or targeting that exposes your campaign to a very large amount of inventory. To respond to the massive volume of inventory on the Internet in the very brief period of time that is provided to respond to a bid opportunity, bidding must be spread across multiple servers. As these servers win bids, this information is communicated back to the server that holds your account's records. The bidders are then periodically updated with information about how much money is left to spend. This happens very frequently (approximately every 60 seconds), however the massive number of bid opportunities means that there can be a short period of time in which some bidders do not know that the remainder of the budget has been spent.
In general, however, SiteScout's pacing algorithm is adept enough to prevent overspend from happening. It is only in cases where inventory targeted is dramatically out of scale with the budget that this will happen. For example, targeting "run of network" with a $5 daily budget is likely to result in a large overspend because SiteScout sees approximately 400,000 bid opportunities per second. It is very likely that in the 60 seconds it takes for budgets to update there will be many impressions won.