What are some results in Economics that are both a consensus and far from common sense? What are some results in Economics that are both a consensus between most economists and far from common sense? 

I would also welcome suggestions of clear definitions for what we should mean as consensus , specially considering that economics is an area with a lot of methodological divergence. Let me try first, a suggested definition for consensus in this setting would be:


  the existence of a group of experts that would claim that the result
  is certainly true.
The principle of comparative advantage

As Paul Samuelson (1969) put it:


  thousands of important and intelligent men ... have never been able to grasp the doctrine [of comparative advantage] for themselves or to believe it after it was explained to them.




Example


  Imagine that an American worker who devotes all his time to soybean production can produce up to 100 tons of soybeans per year. And if he devotes all his time to steel production, he can produce up to 4 tons of steel per year.
  
  In contrast, the corresponding figures for a Chinese worker are 30 tons of soybeans or 3 ton of steel.


Maximum possible production

          American  Chinese
Soybeans     100      30
Steel         4        3


A layperson could reason:


  An American worker is literally more productive than a Chinese worker at everything. So why aren't we simply producing all of our own soybeans and steel?
  
  Instead, we're doing the foolish thing of importing steel from China!


This reasoning is "common sense". It is also wrong.

Although the American worker is "better at everything" (we say he has the absolute advantage in producing both soybeans and steel), the Chinese worker has the comparative advantage (CA) in producing steel. This is because by producing 1 ton of steel, the American forgoes 25 tons of soybeans, while the Chinese forgoes only 10 tons.

And so, by the principle of CA, the American should focus on producing soybeans and the Chinese on producing steel. The two can then trade to mutual benefit.

Numerical example:

Say that without trade, the American spends a quarter of his time producing steel and the rest producing soybeans. The Chinese spends half his time on each. Hence:

1. Consumption without trade

          American  Chinese
Soybeans     75       15
Steel         1       1.5


But they can do better by specializing and trading. The American, whose CA is in soybean production, should specialize in soybeans. And the Chinese, whose CA is in steel production, should specialize in steel.

2. Production after specialization but before trade

          American  Chinese
Soybeans     100       0
Steel         0        3


The American can then trade, say, 20 tons of soybeans for 1.2 tons of steel. End result:

3. Consumption after specialization and trade

          American  Chinese
Soybeans     80       20
Steel        1.2      1.8


Comparing Scenarios #1 and #3, we see that with specialization and trade, both the American and Chinese workers are strictly better off.  Remarkably, each gets to consume more of both soybeans and steel than they did without trade.

Thus, even though the American is "better at everything", the principle of CA offers a powerful rationale for why he should still import steel from China and be "dependent" on the Chinese worker.