We spend an inordinate amount of time ensuring that our clients' web analytics software is installed correctly. We look at page code, we test, we ask the developers to fix it, we test again...and you'd be surprised how often, once it's all working, the code mysteriously falls off a page later on down the road!
I've consulted with our colleagues in the search marketing industry and found that we are not alone. I've also read that up to 45% of Google Analytics installations may be incorrect.
Obviously, if web analytics software isn't tracking correctly, you're not gathering good data, which means any actions you take based on that data may be flawed.
My colleague and friend Brian Ussery has a good blog post about this phenomenon, giving a number of examples of common errors to look out for.
The main lesson: never assume that your web analytics software has been installed 100% correctly - dig through the data and test on a regular basis!