C, like any programming language can be abused. Good programmers can write programs so complex your eyes cross when you look at the code. Sometimes programs get so complex that even their authors can't figure them out. Although these types of programs can be written in any language its just plain easy to do in C.
The International Obfuscated C Code Competition (IOCCC) is an annual contest run over usenet by Landon Curt Noll and Larry Bassel. The contest began when Landon first looked at the code for the Bourne shell. He began to wonder how far you could go if you actively tried to make c code look confusing rather than just achieving it as an accidental side effect.
In 1987 David Korn of Bell Labs submitted this winning entry.
main() { printf(&unix["\021%six\012\0"], (unix) ["have"]+"fun"-0x60);}
Try running that under UNIX. What does it print? (Hint: it's not "have fun"!)
To learn more about obfuscated C visit the IOCCC's website.