Bad arithmetic can leave us like the captain of the Titanic - convinced we're unsinkable while we confidently steam toward the iceberg.
The inability of the Clinton advisors to perform basic number crunching cost them dearly in their primary campaign.
After the expensive loss in Iowa the Clinton campaign focused on states with a primary, such as Texas, pouring an enormous amount of resources into winning them while Obama racked up win after win in states they weren't even running polling in... only to discover that the time, money and effort had gained them little advantage in several of the contests that they focused on. Clinton won the Texas primary, but it didn't translate into a big advantage in delegates. A little number-crunching (which plenty of people pointed out well before the Texas contest took place) would have shown that a big effort in Texas would have conferred a relatively minor advantage, given the way that Texas' system works. But their campaign apparently didn't understand the issue - in spite of the fact that the issue was well understood by others - until too late; the Clinton camp started whining about it a few days before the primary, but it is not like Texas' circumstances were a secret before then.
Clintons' campaign paid "millions of dollars to consultants who offered up dubious advice".
These "experts" then managed to make further, even simpler elementary mathematical mistakes (by applying a calculation suitable for districts with 6 delegates to districts with different numbers of delegates), which meant that time after time, Clinton must have been mispending money, by allocating resources where they would be certain to be wasted and failing to allocate them where they could make a real difference, in effect multiplying Obama's financial advantage many-fold.
[Mathematics was not the only problem in the Clinton campaign, by any means - but it was a very important one that should never have arisen at all.]
What is it that causes monumental errors on the scale of using a calculation based on six delegates - that the target should be 7/12 of the vote ("the magic number is 59%") - for other districts?
Might it be the Dunning-Kruger effect? Is it just arrogance? Is it getting so focused on things like spin and sound-bites that you can't even remember the rules of the game?
Perhaps the Dunning-Kruger effect might also explain why the California Supreme Court have ruled that courts, not statisticians, will decide which calculations are to be used in cases involving so-called "cold-hit" DNA-matches. Statistical experts are to be reduced to "calculators", performing court approved calculations, whether the circumstances merit the calculations or not.
Innumeracy is what lets a political leader spend a trillion dollars on a largely futile and deadly war, and at the same time veto spending a million dollars on an essential education program, with a stunningly small backlash, partly because many voters don't realize the first is a million times as large as the second. Trillion, billion and million all just become different ways of saying "gazillion", and even the most implausible justifcation can be made to sound iron-clad.
A citizen needs enough mathematics to understand such differences in scale, and a political advisor certainly needs at least enough to be able to formulate a sensible strategy. Without it, we're all in for some very painful and expensive lessons.