I've noticed a few threads on flyertalk that mentioned airlines changing seat assignments after ticket purchase and seat choice. This seems to result in a ton of unhappy customers and chaos when people board and try and shuffle back to where they want to be.

I assumed that if you chose your seat - and they didn't change the plane - that you would continue to get that seat. I've not noticed this happening on AC, but I also figured since you have to pay for your seat assignments on certain flights that they wouldn't move you. That said I have never really been picky about seat assignments so its possible that I never noticed - as long as I kept my window seat!

So, does AC do this? Is this a universal problem? Should I start checking my itineraries religiously?

No airline guarantees seat assignments. While they usually don't change, there are reasons that one might change. An aircraft change is certainly one of them. And if a federal air marshal gets assigned to a flight, this could be another one.

However, if you purchase a seat assignment on a Tango fare, and then get moved, I would suspect you would be on very good footing to get a refund of that charge. I'm not sure what official AC policy on that is though.

In my experience seat assignments rarely get changed on AC. When they do they tend to be because:
1. change of aircraft
2. you reserved a bulkhead and got usurped by a family with an infant
3. a family member or the flight attendant politely asks you to move so a family can sit together
4. They issue two boarding passes for the same seat. (yes it has happened to me a few times)
5. Some prick of an SE wants your seat and convinces the concierge to make the change.

Really? Is this a new SE perk?