An ethernet frame with two IP packets inside?

[Keller <keller () wiesbaden netsurf de>]

Hi gurus and beardy wizards, 

what happens if one ethernet frame contains two IP packets?

I know, it *shouldn't* happen, but I could construct one, right?
How will different tcpip stacks deal with the second IP packet?
Could it slip through the filtering rules on some routers?
Could it slip past static pattern matching firewalls (FW-1?) ?

Any ideas or pointers are greatly appreciated.. 

Cheers!

Stefan Keller

p.s.:
I'm aware that it would imply that the attacker sits directly 
in front of the router/firewall server/whatever..
Then again, he could sit on a (compromised) Linux web server 
with .. let's say SPAK.. downloaded to that machine.