The first is that staff who are spending time using the Internet for their own purposes are not working. Lost productivity can become quite significant.
Websense's Web@Work survey of 800 employees across Europe, published last month, showed that 41% of staff use the Internet for private purposes for more than three hours a week. Fifty-two per cent admitted to booking holidays, 41% surfed for education or their hobby and 27% watched sport and used it for shopping.
Some employees use the Net to gamble while others run their e-businesses off the backs of their employers' IT infrastructures.
Ninety per cent of respondents admitted that using the Internet can become addictive.
The impact of all this surfing is also felt on the network. Carrington warns that, with video streaming increasingly common, it can take up a significant amount of bandwidth.
Ironically, it is usually precisely because companies are geared up for large bandwidth, compared with home set-ups, that people prefer to do their movie-trailer watching at work.
If corporate bandwidth is diverted to non-productive uses, says Carrington, other legitimate users can suffer.
"Downloading high bandwidth streaming has a detrimental effect [on network performance] and IT starts getting complaints," he says.
The final danger from surfing on company time is more indirect, but just as real. If staff are using work Internet facilities to download illegal material, such as pornographic, racist or terrorist material, then the company has to beware that it may be implicated in such criminal activities.
In the US, companies are being sued by their employees for creating a hostile workplace.
What, then, should the prudent IT director do about Web abuse in the workplace? Carrington advises, "They have to look at the business objectives when it comes to how stringent the corporate Internet access policy is going to be so that IT can be in sync with the company's goals.
"IT has to look at ways to cut the cost [of Web abuse] and increase productivity, having recognised how critical it has become."
But don't make things too draconian. Internet access policies should be both workable and acceptable - nearly three quarters of the survey's respondents said not only that they thought it reasonable for Internet access to be managed, but they also said they thought it was reasonable to have leisure access in breaks and outside office hours.