The validity of a manufacturer's claims refers to how legally acceptable the claims made on the product are and whether the product, for example, does not contain any deadly toxins or is poisonous. That's all I can think of for now.
I remember it said something like "mixes super-fast in cold milk" and I stood there stirring for about 10 minutes to dissolve just 60% of it. Of course, in most cases solubility increases with temperature.
That was just a simple example. Other ideas that spring to mind are for example concentrated dishwashing liquids that claim to be able to wash a stack of oily dishes with just one drop; and soaps that claim not to dry your skin out.
I did an experiment for year 10 which wasn't quite the same, but similar. I tested 5 brands of soap (ranging from $0.5 to $5 per bar) on skin and fabric to see their effectiveness. Turns out Home Brand is the champion.
EDIT: Gosh, Riv. I thought I'd be the only one here.