[tlug] [OT] freaky network cable issue

["Micheal E. Cooper" <mcooper@example.com>]

Bewildering problem.

I have a room in which half the wired connections work with Mac and Win
clients, but the other half only work with Mac clients. Windows PCs
cannot communicate even get an IP address.

I experimented. I found one of the switches to which these cables
connect. To see if the problem was caused by the switch, I made a new,
long cable (let's call it X) to connect from this switch directly to a
Dell Latitude. It works, so the switch is okay.

I figured it caused by RF interference from power cables or something
under the floor, so to test this, I used a coupler to put the new cable
X between one of the non-Windows-compatible cables (let's call it A) and
the switch. Now it works. So I patch the new cable X in between the PC
and the non-working cable A. It works. I connect the PC directly to the
switch with the non-working cable. It does not work.

All cables are straight.

To summarize:
BAD: PC <-> cable A <-> switch
GOOD: PC <-> cable A <-> coupler <-> cable X <-> switch
GOOD: PC <-> cable X <-> coupler <-> cable A <-> switch
GOOD: PC <-> cable X <-> switch

I am stumped, but kind of amused, since it is like Fun House networking.
Why would adding a second cable fix the connectivity problem?