I recently installed Google Analytics on my site, after seeing it in action at work. One of the reasons I installed it is I was interested in seeing what hits were "real" on my site, as a search engine or referrer spammer won't trigger the analytics Javascript code. (Or, at least the spammers don't seem to be doing it yet.)
I figured that Google would show fewer hits than my web logging software, but I was unprepared for the magnitude of the difference. For Friday, August 4, my web logging software reports 431 unique visiters accessing 1447 pages. (Yup, small site, no?) Google reports 50 visits for 50 page views. That is quite a difference.
I had hypothesized that this site was an unusual combination of well-linked and low traffic, making most of the traffic crawlers and search engines, but I didn't expect it to be that much difference.