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.

The classic is:

  #define TRUE FALSE

or the even more pernicious:

  #define 1 0

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

Might have been the Lua Programming Gems book...