It can happen, if you don’t have redundant systems in place.

Separated in a Metro station, a realization a few minutes later, one person with a cell phone and an address for a meeting, the other person with a key to the apartment.

And never the twain shall meet.

This is how we managed to get into this mess.

–We took only one phone between us, because we were together, right? If either of us needs the phone, we can use it because the other person is standing right there.

–Only one of us knew the name of the cafe where we were going to meet our friend Mary for a drink. Again, one of us was in a “doesn’t need to know situation.”

–And the other of us had the keys to the apartment.

So, the Republique Metro, at rush hour on a Friday night. Heading for the Opera station.

Thing is, two trains that stop at Republique also stop at Opera. That choice would be key in the looming separation.

Walking around outside the ticket gate, looking for a map of lines 3 and 8, trying to divine an advantage of one over the other. (When I think there is none; probably the same set of tracks.)

Then, to the machines that scan your ticket. For the previous two weeks we had not used regular Metro tickets. The little rectangles of cardboard. Instead, we had plastic Navigo passes, which allow the holder access to all Metro stations and to all buses for one full week, at about $20.

The week we had the pass, we used it often, but perhaps not $20 worth it. So it had lapsed, but that didn’t matter much because the group was walking, using buses or cabs.

We had come to no consensus on which of the two lines we would take, though each of us thought we had agreed.

Approach the ticket barrier …

Not all the machines there, and about eight were lined up, left to right, take old-fashioned tickets. And not all take Navigo cards, either.

The former machines are harder to spot than the latter, because they are retro and fewer in number. A commuter looks for a small slot for the ticket in the front of the machine, about three feet above the ground, while the Navigo sensor is a big, unmissable orange spot on the top of the machine.

So, we approached the place, and this was key: the person on the left thought the person on the right, separated by about 10 yards while trying to stay out of the way of the rushing crowds at a five-line Metro, was entering an area where the 5, 8 and 9 trains could be taken, each in two directions.

In fact, the person on the right veered off to enter the area where the 3 train could be taken.

The person on the left struggled to recall where the slot was, for the ticket, and only about three of the machines accepted tickets — and one of them was broken.

A brief wait in line, finally to the machine, the ticket in the slot, retrieving it from another slot, pushing through the turnstile. And by now, the person on the right was heading off for Line 3, assuming the other was right behind, while the person on the left assumed the other was just ahead, heading for Line 8.

When, in fact, we had crossed into a zone inaccessible to the other.

First, the confusion. Two versions of, “I thought you were right there, but I looked around and you were gone.”

Followed by the trip to the platform of a train heading for Opera. Except one was Line 8 and the other was Line 3.

Watching a train pull in, and looking to see who got on. No one recognizable, because we were 50 yards and several walls apart — but still didn’t know it.

The retreat to the common area, where we might expect a rendezvous. Again, impossible, because Line 3 and Line 8 … they have diverged.

A wait of 5-10 minutes, while other facts are reviewed.

One person has neither phone nor any idea where the rendezvous at Opera would take place. Getting to Opera was possible, but then where?

The phone issue could be solved — but only by taking the 10-minute walk back to the apartment.

The other person could go on to the meeting, but could not be sure the other was not lost — with the only set of keys (and two are needed to get into the apartment. A return trip might be fruitless.

So, generic agitation, followed by more up and down, looking here and there, with no result.

Finally, the person with the key headed back to the apartment, fired up the phone, and a few texts later it was clear what had happened. Each thought the other at fault for “assuming”.

But the meet with the friend went off, at half strength, but that was enough.

Anyway, even adults can get lost in the Metro system, during rush hour, and especially at a station where three or four or five (as at Republique) lines all stop.