Carryon bag OK outward bound but not home..

Even with code shares, the luggage rules are dictated by the first flight carrier, not of the entire trip (Air Canada), but by outbound/ inbound flight .....so, yes, you need to check both and pack for the lightest requirement.
Oh, and congrats with the carry on only packing. Don't let this discourage you.

We have always found this to be true, that your luggage, including smaller carry ons, will be weighed and checked more than when leaving the USA.

Code share or not, it's simple: their airplane, their rules. I've had my carry-on weighed by Air Canada a couple of times at check in (usually for super long flights), and have had to prove it fit inside the box a few times as well, so I think you got lucky. Many of the international carriers are much more strict. Best to check this before you leave.

This is such a surprise the first time it happens! My rookie mistake was flying outbound on Delta and not realizing the inbound flight started on Alitalia which yes, had very strict guidelines. I didn't have my charger cords together, kindle was just stashed in the carryon which had to be checked. I had to step out of line to reorganize. It was certainly a learning experience and yes, I'm much better organized now, lol!!

Make yourself some notes on your packing list so you can know what to leave out next time!

Yes, the opearating air carrier defines the carry-on rules. Lufthansa has both size and weight limits for carry-on baggge, with 8 kg the "weight" limit (17.6 pounds).

Or ... going home, just plan to check the bag. If it gets delayed, you can wear the clothes in your own closet.

They said it was far too heavy and I'd have to check it.

Lufthansa limits carryon to 8 kg (17.6 lb). According to the Air Canada website, the only weight limit for carryon luggage is that you have to be able to put it in the overhead compartment yourself.

Austrian also limits carryons to 8 kg, which I was not prepared for. The backpack that I had already checked would have been lighter! The Austrian airlines lady said "I'll let you by this time but don't do it again". Lesson learned and now a fan of Austrian, who also lets coach passengers on short-haul flights pay 15 euros to get the meal served in business class.

I've had the same experience even when outbound and return were the same international carrier. British Airways in the US didn't bat an eye at my carry on bags last year, while their Heathrow crew made me check a bag.

Fortunately I like to carry on outbound and check a bag on my return, so it works ok for me.

I believe that most of the European carriers like Lufthsansa and Austrian allow an 8kg personal item in addition to the 8kg carry-on, so you can effectively bring 16kg with you.

That's in contrast to the discount carriers in Australia (Jetstar, Tiger Air, etc) where the limit is 7kg total including both the carry-on and personal item. If you're going to Australia, bring a jacket with deep pockets!

Any airline can require gate checking your luggage on any flight. That is why when packing for “carryon only” flights it is essential to have all electronics, medications, or items of value packed in your personal item. This way, a last minute gate check doesn’t put those items at risk.

S J , at what point did they weigh and measure it? Was it at the gate, pre-boarding, or up front during check-in?

What CWsocial said. Has happened to me many times on the same airline. Outbound from San Francisco, they couldn’t care less. Returning from Paris, very strict. Another joy of flying.

Yes definitely more common when departing Europe than when departing US. Also North American airlines (which includes Air Canada) are generally more lenient than European based airlines regardless of which country you are leaving from).

But it also has to do with the agent who is checking you in. Last year we were checking in for a TAP (Portugal) flight, departing from Boston. My bag was fine. I stepped to the side while my traveling companion checked in. Her bag was 1 pound (2kg) overweight but the agent said she would let it go. On the opposite side of me was someone checking in with the next agent and her bag was also 1 pound over and that agent was making her check it. So sometimes it just has to do with who you get. But if you want to be absolutely sure you can carry on your bag make sure it is within their size and weight limits. I sometimes take out a few heavy items (ipad, phone, chargers, small camera, etc.) and put them in my jacket pockets during check-in. No one has ever noticed and by doing so I can reduce the weight by 1-2 pounds.

It was when I was checking in at the Lufthansa desk in Florence.
The agent said my personal items could not weigh more than 8kgs total.
Air Canada could not have cared less!

Rules can change! One time I booked a flight months in advance and read the baggage rules. Then, sometime before the flight - the airline changed its luggage rules. We were following the original rules for packing.
So, lesson learned. I check luggage rules about 1 week prior to flying.
My family and I have accumulated an array of bags/packs (admitted bag addict). So, I do the "math" and assign various bags as required by the airline's rules. Luggage scale helps at home.