As notzed said:

For example, if you add a random sequence:

2^-30, 2^30, 2^-30, 2^30 ...

you'll end up getting the same as:

0, 2^30, 0, 2^30 ...

because the mantissa of a float isn't big enough to hold the extra bit of precision.

However, if you sorted the list:

2^-30............... 2^30....

You would be able to add pairs of 2^-20 and if you add enough the sum may be enough to affect the addition with 2^30, and the result could be significantly different given the right combination of values and length of the list. That's an extreme case, in the general case any operation of floats incurs a possible loss of precision, and the order in which you do the operations changes the particular information that is lost.