For those skeptics who worry that an athlete could easily spike a container of an uncontaminated supplement and lay the blame there, it's not that simple. You need to show contamination on a sealed container. Further, the level of contamination needs to be consistent with the amount found in your system. Both in terms of timing and amount. The math needs to add up. It's not just about showing that a given supplement was contaminated. It has to be mathematically plausible that the contaminated supplement caused your AAF.
According to Howard, NADOs have not, to his knowledge, used other metabolites (from allowed substances in a product) to confirm that you actually took a given supplement in the stated amount or even at all. They rely on the math of the testing; if you knew all the details about how to present a plausible scenario for supplement contamination in advance, you'd know enough not to get caught in the first place. And, of course, this again speaks to the traditional investigative aspects of these cases. How believable - or not - is a story? There's no way to quantify that, and yet it is important. But, to answer the question posed on the forum, an athlete doesn't have to prove that he actually took something. He should have listed the supplement on his TDU form (Therapeutic Declaration of Use), but this only covers the prior 72hrs.
While some NADOs have interpreted that saving bottles constitutes an acknowledgement of the inherent risks of supplements, according to Howard, no NADO has successfully used that argument in arbitration against an athlete. Howard recommends saving at least the bottles/containers of any supplements used until after a confirmation of a "no result" from a given test. While the list of what constitutes a supplement is certainly more broad than many folks realize, I don't believe that you need to save everything. While I don't think you should use gels in disposable wrappers for environmental reasons, I also don't think the industry is so unregulated as to necessitate your saving all of your gel wrappers either. If you don't view winning the lottery as a reasonable retirement plan, I don't think you need to hoard your trash either.
Ultimately, you have to have some faith in something, and I choose to place my faith in math and in due diligence. If you do a bit of research and avoid bottles that look like the cover a comic book, you're way ahead. There's no reason with the ease of conducting research on the internet and with the third party certifications available that any athlete should ever have an AAF from a contaminated supplement.