I mean, as the sibling notes, there's redefining true as false, but I think making all structs unions is much funnier, especially in that it's a uniquely C proposition. There are other languages that allow redefinition of true and false.

Also, I'm not sure how you think it's particularly obvious (as opposed to any other #define trick). It does only work if the project isn't warning-safe, as there are various ways this'll trigger ugly warnings (eg. using designated initializer syntax causes re-initialization warnings), but even then the warnings don't tend to actually tell you it's a union all of a sudden.

If you have others, I'd like to hear them.

- Change the alignment of structs

- Remember the order of includes change how they are applied. Have a #define that changes something in a .h that's included in some files but not in others

- (Depending on the software) redefine rand() to be almost predictable or with a short loop

  #define if while
  #define else while(!true){}

That should be interesting.

The classic is:

  #define TRUE FALSE

or the even more pernicious:

  #define 1 0

I once chased a bug which turned out to be: someone had #defined the letter 'T' as something else, in a chain of #include files nested 19 deep. Completely screwed up a bunch of C++ templates.

Straight from the Ministry of Truth that one...

I read somewhere about someone redefining goto as goat, but I can't remember where.

Might have been the Lua Programming Gems book...