I will confess, I fly AC but I try to collect United points to keep up with the Jones' (1K, in this case). Problem is AC doesn't want me to collect Mileage Plus miles on their flights, only Aeroplan. For the last few yrs anytime I use my MP account #, it tells me to go away. OK, it tells me I cannot check-in and I need to see an agent. Grrrr! At one point Web support told me to only use my Aeroplan account number for booking and checking in, and then change it at the gate. Seems I was placed on a 'list' by AC HQ and if I use anything but my Aeroplan #, they have to verify me vs another naughty 'canucklehead' who may or may not be a bad guy (I know I am not a bad guy, cuz my mother told me so and she never lied! ) Instead of getting hassled, I have used my Aeroplan acct# or fly other airlines for my short hauls to Canada (Porter, UA - via IAD ). Thought I would try this time to change account #s at the gate. Big mistake -- I felt bad for the agent as he kept on typing, and typing, first my UA #, then Aeroplan, then UA...looks up at me, smiles and says "do you know that your name is flagged?" I give a short description of my past travails, ask him if he needs to call AC. Yup! Spend next few minutes giving my info....finally gets ok to use my UA #. Anyone else have this problem, any resolutions?