I couldn’t find a good simple explanation of what this error means on the web, so to future googlers, you’re welcome.
This most likely means that the user made a web reqeuest, probably through ajax, on your site and then cut off the connection before the ajax request completed. The connection can be cut off by clicking on another link, closing the browser, or things in the middle like firewalls and whatnot.
There isn’t a good solution to this issue that I could find, the error is just something that happens from time to time. If it happens regularly and you don’t think it’s an accidental connection cut off, do some more exploring (particularly with firewalls and other potential connection problems), otherwise it can be ignored.