When you sign up to google analytics it instructs you to use a javascript snippet on every page you want to track. This code includes an API key, which is visible to everyone who views your source code.
How does it guarantees that the request is coming from the real site, and not from a third-party who wants to mess with your statistics? Does it check the HTTP Referer header? Even that is not safe, as it can be forged.
GA doesn't (to the best of my knowledge) attempt to verify that the site ID (the UA-XXXXX-XX code) matches a domain specified in the GA setup - I think this is a good thing, as you can track a bunch of related sites as though they were a single site (think single-product minisites, for example). However, this does leave the GA profile open to accidental or malicious use of the UA code on other unrelated sites.
The easiest way to fix this is to add a filter onto the GA profile which restricts reported data to a specified referrer hostname set. This will clean out the accidental typo problem, but malicious types would be able to work around this if they were really interested (but they'd be more likely to grief your PPC campaigns instead).