Why is -freciprocal-math unsafe in GCC?

-freciprocal-math in GCC changes the following code
double a = b / c;
to
double tmp = 1/c;
double a = b * tmp;

In GCC manual, it's said that such an optimization is unsafe and is not sticked to IEEE standards. But I cannot think of an example. Could you give an example about this? Thank you.

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