Last year the Air Canada check in desk weighed my carry on bags (and then sent me away to reduce weight!) I had lunch, a bottle of coke (which was consumed before security) etc and one bag was 8kg, the other 12kg after food and drink removed. Was that ok? No. I had to repackage items so they were exactly 10 and 10. I wouldn't have minded so much if I hadn't seen people with 3 or 4 carry on bags, some containing the kitchen sink by the looks of it!

On the plus side, BA don't have weight limits for carry on, the only restriction is that you can lift it. The disconnect between carriers does cause considerable confusion.