In the world of mobile computing, battery life is a key issue, and one that is difficult for manufacturers to tackle from a variety of angles. Bumping up the performance, for example, usually has an impact on battery life. Thus, being able to market a long battery life while still keeping performance and weight in-line with the competition is a good thing - and PC makers certainly know this.
When Acer recently demoed their 10 inch Aspire One netbooks, many reviewers were pleasantly surprised to discover a netbook that could boast an uptime of nearly 8 hours while under moderate use. That was largely made possible by the battery capacity, which was a six-cell 5800mAh unit. Unfortunately for potential owners of the netbook, the units that reviewers (and some early bird customers) got were "mistakenly" shipped with the larger battery.
Most of the units will actually be delivered with a 4400mAh battery which would in turn have a shorter runtime. Acer claims the smaller unit will still get around 6 hours of battery life, though one review site speculates it'd be lucky to get 5.5 hours, compared to other netbooks with similar sized batteries and specs.
While an early shipping mistake isn't that big of a deal, it can easily be made one if it is perceived by the public as deception. Did Acer intentionally ship higher-specced systems to reviewers, only to boost the review? I would give Acer the benefit of the doubt in this situation, since the company is apparently not selling the higher-capacity battery as an upgrade and they likely have other plans for it.