Gaining

I'm curious whether anyone here has ever ironically used a product intended to help people lose weight to instead gain?

For example, I recently picked up one of those small 600ml 'smoothie makers' which promotes itself as being 'ideal for making delicious healthy smoothies before work, school or the gym' to instead 'make delicious fattening milkshakes, slush puppies and cocktails'.

I had a blender, but it was heavy and a PITA to use and clean for 1 person and 1 session.

Frankly there are a lot of "healthy" products that seem to me to be likely to cause weight gain.

Smoothies and the like always seem like a trap. Processing food like that so they're easier to digest sounds like a recipe for a soon-to-be empty stomach and increased cravings later on.

Then, artificial sweeteners have been shown to trick brains into increasing caloric intake through increased sugar cravings as brains decide they need to increase our desire for sweetness to make the calorie intake match the perceived sweetness. When actual sugar is encountered, the result is eating way more sugar than one would before an extended diet including "diet" sodas.

On the flip side, despite its reputation of giving people munchies and causing them to laze about, marijuana consumption is linked to lower BMIs when comparing non-consumers with consumers and when comparing those who have only started weed to those who have been using weed for a long time. This is despite the increased caloric intake of people who consume weed:
www.ncbi.nlm.nih.gov/pmc/articles/PMC6340377/

Biology is weird, man.

I was thinking more of intentionally 'misusing' to gain rather than accidental.

Oh for sure, but there's a very very thin line between accidents and intent, once you're aware of those hidden effects.