Running splint ( http://www.splint.org ) over this code:

---8<----------------------
unsigned char x,y;

void main(void)
{
  x^=y^=x^=y;
}
--->8----------------------

results in this message:

xy.c:5:6: Expression has undefined behavior (value of left operand x is
             modified by right operand y ^= x ^= y): x ^= y ^= x ^= y
  Code has unspecified behavior. Order of evaluation of function parameters or
  subexpressions is not defined, so if a value is used and modified in
  different places not separated by a sequence point constraining evaluation
  order, then the result of the expression is unspecified. (Use -evalorder to
  inhibit warning)

Ooops, I'm surprised as I did not even hope for such a detailed message!)