Should BODMAS not be BODMSA? Let me blindly follow for a second BODMAS.
$$
\begin{align*}
1-3+2
&amp;=1-(3+2)\\
&amp;=1-5\\
&amp;=-4
\end{align*}
$$
(The brackets are just to make the error clear - I wouldn't write them in "real life".)
This is correct according to BODMAS, because BODMAS doesn't talk about positive or negative numbers. It just says that addition should come before subtraction.

What am I missing here? Is it like "i before e except after c", and then you realise their is a word and the whole rhyme is as follows:


  i before e,
  
  Except after c,
  
  Or when sounded as "a,"
  
  As in neighbour and weigh.


So, am I missing some subtlety? Is the rhyme incomplete? Is it not true that subtraction has greater precedence than addition, and so BODMSA is actually correct and BODMAS is incorrect?

NOTE: This issue is true for all other mnemonics I know of, such as BIDMAS, PEMDAS, PIDMAS...
In the Danish educational system we are taught the order of operations in the following hierarchy:


Brackets
Roots and powers
Multiplication and division
Addition and subtraction


along with the rule of carrying out operations of equal order in the reading direction. In effect we have a BO(D,M)(A,S) system where (D,M) and (A,S) are pairs of equal order. This yields the same results for any calculation as all international standards prescribe, so no need to worry. The funny thing is that in our system you are meant to calculate as follows

$$
a\cdot b/c=(a\cdot b)/c
$$
whereas the English BODM(A,S) prescribes
$$
a\cdot b/c=a\cdot(b/c)
$$
Of course those two expressions are equal to one another, so it produces no controversies about Danish and international order of operation standards.

My best guess is that BODMAS was ordered that way in order to form a pronouncable word rather than the tongue twisting BODMSA knowing that A and S are equal as long as you apply the rule of the reading direction too.

NOTE: Actually PEMDAS would cause problems too unless we interpreted it as PE(M,D)(A,S). For instance $a/b\cdot c$ would equal $a/(b\cdot c)$ if multiplication really came before division. But in reality what is meant is reading from the left to the right performing operations of equal order, namely $a/b\cdot c=(a/b)\cdot c$.
It must be BODMSA at all times. BODMAS is fake. Don't try to defend it and reject the impeccable BODMSA on the grounds of pronunciation. Can the 'nice' pronunciation right the wrong?
Someone suggests we teach our students to consider, for example: 12-5+4 as 12+(-5)+4 without regarding the minus sign. I ask my fellow teacher, "what about 12-(-5)+4? Is the minus sign still insignificant?
In fact, we must admit to give priority to subtraction over addition in the order since it is correct at all times, whether it be at the left or the right. 
BODMAS says 12-9÷3+5=4; also 12-(-5)+4=13 while BODMSA gives correct answers of 14 and 21 respectively. 
It's true doing the operations from left to right is correct doesn't mean we should embrace what is not true.