TITLE: Strangest Notation?
QUESTION [14 upvotes]: While this may be a fruitless pursuit of anecdotes, I still ask: what is the strangest (or most blatantly wrong (at least in the eyes of common notation)) mathematical notation you have ever seen?

REPLY [9 votes]: The Landau big-$O$ notation is extremely strange.  

One writes  $$f(x) = O(g(x))$$ which looks like $f$ is the composition of $O$ and $g$, but it is nothing of the sort. Is $O()$ an operator that can be applied to any term?  Can I write $$O(x^2) = O(x^3)$$ or $O(x^2) = 2x^2$?  Not normally.
It is easily confused with a whole family of similar notations for similar notions; computer programmers regularly talk about $O(n)$ algorithms when they mean $\Omega(n)$ algorithms, for example.   This is exacerbated because someone decided that instead of using mnemonic abbreviations, it would be a good idea arbitrarily assign every possible variant of the letter ‘o’ in naming them.  Then when they ran out of letter O’s they used $\Theta$, seemingly because it looks enough like an O that you might confuse it with one.
It is written with an $=$ even though the relation is asymmetric!  We have both $x=O(x^2)$ and $x=O(x^3)$ although $O(x^2)$ and $O(x^3)$ are not the same, and we have both
$1 = O(x)$ and $x = O(x)$ even though $1\ne x$.