What is the most useless warning label?

It could be effective, but opponents couldn't kill the initiative, so they made it ineffective by ensuring the threshold was set so low that essentially everything qualifies.

The most effective marketing bullshit that's been achieved in the modern world is the misinformation McDonald's put out about that incident to try and discredit their active negligence. Making people think that the hot coffee lawsuit was frivolous.

The coffee was so hot, 190F that can cause 3rd degree burns in less than 3 seconds... which is exactly what happened. The woman had 3rd degree burns over 16% of her body. The car wasn't even in motion, the lid popped off the cup. She was only seeking restitution for her medical bills, $20k but the company refused, hence the lawsuit.

McDonald's had been holding their coffee at unsafe temperatures so they didn't need to toss it and brew new coffee as often, saving tiny amounts in the process, because coffee is not expensive, especially the crappy coffee they used in those quantities. They had been warned over 700 recorded times (so who knows how many in reality including those unrecorded) that holding the coffee at those temperatures was a safety issue, and they actively chose to ignore that to save pennies.

The jury found the company 80% liable and awarded $200,000 in compensatory damages and an additional $2.7 million in punitive damages because of McDonald's negligence.