I recently investigated a bug which caused a page load in IE7/8 to make two requests to the server, the second one without a querystring for some mandatory parameters – which caused some warnings in the server log.
Turns out that image tags with an empty src attribute cause this to happen in at least IE7 and 8. So if you have such a thing on a common page elements such as a header or footer, the load on your servers is doubled – not good.
There’s an in-depth discussion of this over at NCZ online – Empty image src can destroy your site, and also a solution using the base tag to prevent this from happening in IE.
The best solution of course is not to have such tags in the first place, but it’s good to have a defence given how easy it is to never notice the extra request.