I've been had by Mr. Bender's lectures

Bamboozled, duped, hoodwinked, well, you get the idea. I drank his cool aid on (lack of) mathematical rigor and I bought his idea of summing infinite series like

$1+ 2+ 3+ \cdots = -1/12$

What I did wrong was swallowing hook, line, and sinker his postulates:

• Rule 1: Summation property:
• $S(a_0 + a_1 + a_2 + \cdots )= a_0 + S(a_1 + a_2 + \cdots )$
• Rule 2: Linearity:
• $S(\sum (\alpha a_n + \beta b_n) )= \alpha S(\sum (a_n)) + \beta S(\sum (b_n))$

Why? Because in addition to assuming a unique result, they are inconsistent. If:

$1+2+3+ \cdots = -1/12 = A$

Then for example consider this:

$A-A = (1+2+3+ \cdots ) - (1+2+3+ \cdots )$ 

which by rule 1:

$0 = (1+2+3+ \cdots ) - (0+1+2+3+ \cdots )$ 

such that

$0 = 1+1+1+1+ \cdots$ 

Aha! This now implies that

$0=1 + (1+1+1+\cdots ) = 1+0$ and so $0 = 1$!!!!

The two rules work for alternating sums, but when the sign of the sum terms is the same the two rules are clearly inconsistent. 

But does this mean that $1+2+3+ \cdots$ is not $-1/12$ ? Not at all. The result is still valid due to deeper reasons: analytic continuation of Riemann zeta function.

It is not easy to find why things like this work in math, but in general physics intuition is a very good clue that there must be a solid and rigorous foundation. It is just that physicists' focus is on solving the practical problems and not on the deeper mathematical theory. One may say that a physicist to a mathematician is like an engineer to a physicist :) This is not that bad though: the engineers make more money than physicists, and physicists make more money than mathematicians. 

I still regard Mr. Bender's lectures as outstanding, but I should have trusted my mathematical intuition more and not disregarded the alarm bells of mathematical rigor. The inconsistent argument above is due to David Joyce and I ran across it on Quora where the -1/12 result is discussed often.