No experience with an expired carnet, ours were still valid upon entry and exit. As you know, Egypt is one of the countries still causing the need for carnet due to extermely high import taxes and lack of alternative temporary importation permits that many other countries have adopted as an alternative.
Having said that, it is a very bureacratic country so if it is going to get noticed and picked up on, it wouldn't surprise me if it happened in Egypt. It depends a lot on the border you are exiting and the specific agent. If you are leaving through Aswan, it's a much smaller office and there are only a few that they see so I'm guessing your chances are better that they won't even look at the date. Coming in on the ferry from Jordan, the entry process was pretty long and there were a lot of steps and they see a lot of vehicles in and out so if that is where you are exiting that might increase the odds that someone would notice.
If they do notice, they would almost certainly be some discussion and effort to extract some cash out of you. When we exited Jordan, on some of the bikes one of the documents was stamped with the wrong date. After much arguing they extracted a couple of bucks off us to correct the document and off we went. I'm guessing the Egyptians would be more aggresive in this regard.