Well, the highly improbable finally happened today.

 I got an e-mail from work saying that one of our (old, but just newly plugged back in after months of non-use) printers (a Star Micronics product) wouldn't come up on the network.  I said I'd check it out when I got to work.  Expected nothing other than maybe it was just configured improperly.

I tried the normal setup procedure, no go.  I printed out the test page, showing that it was indeed picking up DHCP.  Couldn't connect to that address.  Tried manually setting the arp table entry.  No go.  Pinged the broadcast address and displayed the arp table.  Lo and behold, there were TWO IPs with the same MAC address!  Switched the printer off and tried to telnet to both the IPs.  Nothing.  Tried browsing...  Well, what do you know, that old Netgear switch (10/100 only, servers just some old slower equipment we have) sitting in the closet comes up!

Ended up disconnecting the Netgear switch and tossing in an equally old Linksys 10/100 (unmanaged) switch that we picked up cheap awhile back.  Plugged the printer in, and everything configured just fine!

My understanding of the Ethernet standard is that EVERY device leaving the factory is supposed to be stamped with a supposedly universally unique 48-bit MAC address.  I could imagine it's possible that on rare occasions a pair of identical devices may accidentally get the same MAC applied to them.  To have 2 completely different products from completely different manufacturers end up at one little company (single digit number of employees, only 2 switches, 1 router, 6 (or so) workstations, and 5 servers on the network!) is absolutly mind-boggling!  The old saying about the improbable is apparently quite true...