"It's a banana Michael. What could it cost, 10 dollars?!"
You say "obvious" but I don't think that means what you think that means. Why is it obvious? Why would it be impossible that a store sells bananas at a loss in order to pull people in (what people in sales call a "loss leader")? If you don't buy bananas regularly, why couldn't you assume this is just a great deal?
If there were a sign advertising 5 bananas for a penny (or whatever) with no asterisk, fine print, or mention that "terms may apply", I would be livid if I couldn't take advantage of that price.
If you would like links: Here's a 5 year old piece about Amazon giving away bananas: https://www.foodandwine.com/news/amazons-free-bananas-disrup...
Here's a 2 year old coupon for free bananas from some couponing blog: https://www.thecouponingcouple.com/free-bananas/
Another blog, more free banana coupons: https://spoonuniversity.com/how-to/10-food-budgeting-tips-i-...
TL;dr- Don't put up bullshit pricing signs and assume people will understand the "joke", you're coming from a place of different cultural awareness and it will burn you when people like me get involved.