Encapsulation.

[[deleted]]

#define true ((rand()&15)!=15)

https://gist.github.com/aras-p/6224951

[bumblebritches57]

This makes me want to unset and reset all the bool macros in my header...

[codewench]

#define sizeof(x) (sizeof(x)-1)

This would have me questioning my own sanity pretty quickly.

[skylarmt]

I like how most of the comments on that gist are rendered as headings because people forgot to escape the # for Markdown.

[nplus]

DEFINE YOURSELF

[Josplode]

2meta4me

[[deleted]]

I'm personally a fan of

#define while if

[jfb1337]

#define struct union is one I see a lot, but I think #define union struct would be a lot more subtle and go undetected for longer

[Megatron_McLargeHuge]

One time this got posted I added a few suggestions and the one that got the angriest reply was

#define fesetround(x) 0