There is a pretty nasty bug in Magento if you are using multi store view with configurable products. I have recently patched it nicely with an extension that overrides the original controller method. This is a much nicer fix than actually hacking the core files which can then break in the next upgrade.
Magento Store Views Archives - ACTIVO
This is a simple fix that makes a big difference. For some reason it is set to show up by default, however some websites that are sensitive to SEO or want to avoid this parameter so you do not get any weird values in Google Analytics or any other Analytics that you may be using, here is how to remove the ‘___from_store’ query string: