Last month I was trying to book a flight in LX F through Aeroplan, and was told by the agent that there are no "O" class seats available on the flight I wanted as per her display.

However, in Sabre (using the direct access function), I was able to see O1 (or 1 seat available).

Seems like AC's reservation system does not show updated availability on other airlines (and their display is not updated instantly when cancellations/inventory reductions occur at the other airline). As such, sometimes agents would get "unable to confirm" messages back as a result of their display showing seats availability (but in reality none are), or vice versa.

I had to tell the Aeroplan agent to do a "long sell" entry by typing the flight segment in the "long way" (I gave her the example that in Sabre I would type 0LX138O12MARZRHHKGNN1 instead of the usual "01O2" [sell 1 "O" seat from line 2 of availability display]) and ignore what she sees on her availability display.

She tried it and told me that the segment came back as "PENDING" instead of "HOLD SECURED" (which I knew would happen anyways), so I tell her to "end the record". Low and behold, a few seconds later after she kept refreshing the PNR (by "ignoring and retrieving the record"), the segment came back as "KK" (or confirmed by LX's system). So in order to issue the ticket, she had to change the segment status from "KK" to "HK" (which would have happened automatically if the usual "short sell" method was used), and then input the connections indicators into the ticketing record and collected the taxes.

The agent even commented that I should work for Aeroplan