Strangest language feature

What is, in your opinion, the most surprising, weird, strange or really "WTF" language feature you have encountered?

Please only one feature per answer.

---

In C, arrays can be indexed like so:

a[10]

which is very common.

However, the lesser known form (which really does work!) is:

10[a]

which means the same as the above.

---

In JavaScript:

 '5' + 3 gives '53'

Whereas

 '5' - 3 gives 2

---

In JavaScript, the following construct

return
{
    id : 1234,
    title : 'Tony the Pony'
};

returns undefined is a syntax error due to the sneaky implicit semicolon insertion on the newline after return . The following works as you would expect though:

return {
    id : 1234,
    title : 'Tony the Pony'
};

Even worse, this one works as well (in Chrome, at least):

return /*
*/{
    id : 1234,
    title : 'Tony the Pony'
};

Here's a variant of the same issue that does not yield a syntax error, just silently fails:

return
    2 + 2;

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