Here's a new one.

Tried to book a flight on a pass bought in August. I'd already booked four tickets on this pass.

Today, tried, and got an error message "Unable to book due to invalid frequent flyer number" and "Error code 820"

On the phone with Website assistance while they try to figure it out. Seems to be some type of name mismatch. I tried again, and instead of

Mr. First IntheAirGuy

I buy it as

Mr. First MiddleName InTheAirGuy

which now precisely matches the details in my Aeroplan account ... and the booking goes through.

The fact I didn't have to do this with the previous four bookings on this flight pass would suggest to me that AC recently tightened up the code on pass bookings, and PRECISION might now be absolutely necessary ... ie if you have a middle name, ya' gotta use it.