A browser bug can cause a request for the text portion of a page twice, but the graphics only once. The impact on the ad impression count depends on the tracking software. Rogue bots are rare. Internet bottlenecks can cause the visitor to request the same page repeatedly until the full page with graphics is actually displayed. But the traffic counting software is counting all requests for the full page, thereby overcounting impressions.
Bottlenecks affect larger banner ad graphics more than smaller banner ad graphics.
If a banner ad is large, it takes longer to load, giving the visitor a chance to move on to another page or site before it is displayed - yet the impression is still counted. Smaller banner ads may load faster, ensuring the visitor sees them, but they may be too small to capture the visitor's interest. For these users, a text advertising message is delivered instead of a graphic banner ad. Advertisers are more frequently demanding that they only pay for ad impressions when the actual banner ad graphic is fully loaded, not just the alternative text version. Requiring the visitor to navigate a site using only graphics and not text links can significantly improve this problem.