What I thought of myself when I read this post was the expression, "self-fulfilling prophecy," a figure of speech meaning that a forecasted event occurs even despite when (and more specifically, because) people take action to try to avoid it.

This comes from the term "self-fulfillment," the concept that the revelation of a prediction about an event itself causes the said event to occur exactly as predicted. I sense that I may have skewed from your idea, though.

For your exact idea, "perfect storm" is a perfect expression. It also may be called a "butterfly effect" and is related to the philosophies called consequentialism and teleology.

First, there is no word "underexaggerate." What you're thinking of is "understate." To say you have a mere flesh wound when someone has fatally stabbed you is to make an understatment (not an underexaggeration). You also can't underenlarge things or underincrease your paycheck.

If you can have an apple, a big apple, and a really big apple, does not mean that you can underexaggerate, exaggerate, and overexaggerate. "Exaggerate" is fundamentally over-the-top, while apples are not. "Exaggerate" is not a neutral word in this way. It describes the act of over-enlarging in a certain sense. You cannot overly over-enlarge.
"There were a thousand people in the elevator" is an exaggeration. "There were a million-bazillion people in the elevatir" is also an exaggeration, not an over-exaggeration. There do not need to be degrees of exaggeration because the word in itself includes doing an action "to a degree that is already higher than what is normal or usual."

I'm very surprised by several contributions proclaiming that the understood pronoun is "I," when in almost every usage of the imperative mood (commands), the understood pronoun is "you."

"Don't mind if I do" is the prescriptively correct way of giving the polite command, "Don't (you) mind if I do." The subject is "you." This goes for most commands: Be my bride! (Be [you] my bride!); Eat your dinner (Eat [you] your dinner); etc.