Ive posted this here in a few places, but I ll go ahead and post it again.
The newest version of many browsers are now shipping out with Block 3rd Party Cookie automatically enabled. The majority of people will never go into their settings and change this back to Enable All Cookies.
This means that cookie stuff will not work. This also means that a forward/mask, which is the same as a iframe on a index page, will not load cookies and track your sale if the Block 3rd Party option is checked in your browser.
I have tested this out and i know for a FACT this is real for the new IE and the new Safari and I believe to be true for the new Opera but Im not sure on that. I know that Chrome and Firefox both have not done this yet and both have allow 3rd party cookies enable automatically........but that is right now. They might follow the other browsers in their next release.
Your safest bet it to just forward to the offer and not mask it.
Also, I can tell you that affiliate networks have encouraged me to do this...especially when you are doing ppc and it requires a legit URL.