Example Heterogeneous Networks -

Even on IEEE machines results may differ between machines, compilers and compiler switches.
An iteration where the stopping criterion depends on the machine precision. Stopping criteria for iterative methods -- may not be satisfied on each processor simultaneously
Processors sharing a distributed vector, v, compute its two-norm, and depending on that either scale v by a constant much different from 1, or do not.
Bisection for finding eigenvalues of symmetric matrices.
Eigenproblem for a tridiagonal matrix -- run QR on each processor and each processor finds k eigenvectors. But each processor may compute a different QR sequence
Adaptive quadrature -- [a, b + e_1], [b + e_2, c]

http://www.netlib.org/utk/people/JackDongarra/