The example code below, compiled on x86 with gcc versions 4.1.2 through 4.4.0, aborts when executed.

#include <stdlib.h>
#include <assert.h>

int main(void)__attribute__ ((optimize("O1")));  // Also with '-O2'
int main(void)
{
   const float a = rand(); // Any function call that returns 1804289383 will do
   volatile float tmp=a;

   assert(tmp==a); // This one aborts
   assert(tmp==a); // This one passes (if line above is replaced with printf)
}

If you replace assert() with two consecutive printf("%f ", a), two different values are displayed for the constant variable a.