OT: Incoming/Outgoing route of IP subnet

[Alexander Steinert <stony8@gmx.de>]

[Hoping that I didn't enter too many killfiles, let me say that I'm
very sorry for not providing a subject in my last mail!]

I've got a question for all the network administrators out there with
more practical experience than I have:

OK, let's assume we have
an IP subnet N,
a host H in N,
a host G in N and
a host A outside of N.
Packets from H to A go via G, since H is configured as gateway on H. 
How save is it to assume that all packets from A to H (the other way
round) go via G?

I'm looking real world examples where a packet enters an IP subnet on
one way (interface) and an answering packet leaves N on another way
(different interface). Only load-balancing or dedicated firewalls
come to my mind. Is such an environment found often? Are the other
examples?

TIA

Stony