Mathematically Annoying Advertising:

[[A ∪ B = {x:x ≤ 15 or x > 15 } = ℝ ]]
[[line graph representing the above equation]]
When discussing real numbers, it is impossible to get more vague than "up to 15% or more".

[["FREE*" in large text, with substantial illegible fine print]]
If someone has paid $x to have the word "free" typeset for you and N other people to read, their expected value for the money that will move from you to them is at least $(x
(N+1))

[[graph representing inverse relationship between "amount you spend" on the y axis and "amount you save" on the x axis]]
It would be difficult for the phrase "the more you spend the more you save" to be more wrong.

{{Title text: I remember the exact moment in my childhood when I realized, while reading a flyer, that nobody would ever spend money solely to tell me they wanted to give me something for nothing. It's a much more vivid memory than the (related) parental Santa talk.}}