[CentOS] random roundoff?

On my CentOS 5 box, in a C++ program that does much arithmetic,
including numerous matrix multiplications, I have a situation in
in which the result depends on the nature of nearby I/O.  Thus,
with all arithmetic done with type double, and where values
are mostly in the range [-1.0e0,+1.0e0]  or nearby, I do:

   cerr << "some stuff" << endl;
   mat3 = matmult(mat1,mat2);

I get a difference of the order 1.0e-15 depending on whether the
cerr line does or does not end in "endl" as shown.

I am imagining that there is some "randomness" in the roundoff
that depends on the I/O situation.  Is this credible?  Any other
suggestions?

Thanks for your help,
Mike.