I was commuting in uniform on a CRJ200 (one of the types on my ATP but not my own metal), sitting in the last row, 2 days before Christmas. So, it was cold outside. It was really hot in the back of the cabin, excessively so. I mentioned to the flight attendant to say something to the captain because it did not seem normal. She returned and told me that the captain said it was normal and it would cool off after the engines were started. Now we are taxiing and it is getting hotter. I hailed the FA again and told her to tell the captain that the PACK is overheating, that this temperature is not normal. She returned once again and told me that the captain said it was normal and that it would cool off once we took off. I stressed a third time that I believed that there was a problem with the PACK and the captain should do something about it. The FA basically ignored me on the third attempt. We take off and before reaching 1000 feet smoke starts pouring into the cabin, LAV smoke detector starts beeping, and the captain comes over the PA to tell us that we are returning to [airport] because of a PACK overheat.

Since I was in the last row I was the last off of the aircraft once we returned to the gate. The crew were standing in the front waiting for me. The captain made it a point to tell me he learned his lesson that day and that he would never disregard a report of a problem from a jump seater/dead header again. The FA apologized for not being more urgent with the captain.

The point of my story is, I usually don't say anything unless I believe it to be of an urgent nature. I can only assume that other dead headers/jump seaters operate the same. So if I have a pilot in the back of the aircraft telling me there is something wrong, I would be a fool not to address it immediately. What is the harm in being overly cautious in that scenario? The worst that could happen is a trip to the CP office to justify a delay. And I would have no problem justifying a delay because another professional expressed a concern with the airworthiness/safety of the aircraft and that I took it seriously enough to return and have it addressed before departing.

ps. I might not say anything if it were a broken latch on an overhead bin, but ice/snow/freezing precipitation adhering to the wings or a potential PACK overheat, you're godd*mned skippy I will say something.