This is one of them questions that can be open to all sorts of speculations as there are so many variables involved, i.e how much shot is running through, at what temp, fuel to nitrous ratio blah blah blah. Im no guru on this but Ill try and answer with my very limited knowledge, anyone else feel free to correct me if Im out.
In order to answer your question we must first understand what a nos kit (or correctly said a nitrous system) demands from the engine of a 350z and how a miss shift can effect it and under what conditions.
When we look at a nitrous system for the 350z one of the most overlooked questions that needs to be asked is what is my limit concerning Torque and Not so much BHP. The reason why you have to understand Nitrous torque is because it is relative to the RPM that you hit it at. The torque from NItrous is to say the least... CRAZY and I certainly would not recommend hitting the nitrous below 3000 RPM otherwise yes depending on how much power you have you will damage the engine over a short period of time or fault it completely.
So in answer to your question if you missshift and the result drops the Rev below 3000 rpm and you have nos freely flowing then yes it is possible to ruin the engine....
BUT !!!!!!!!!!!!!!
And its a big but, there are 2 things worth mentioning that should put your mind at ease......
1.) Buy a RPM window switch
2.) You have to have a controller
Explained:
1.) RPM Window switch
A RPM window switch tells the nitrous to only turn on between certain RPMS, you can set these to start at whatever RPM you would like. I used between 3500 to 6000 RPMs on a 55 plate, a little higher on the rev ups. This give you the safety of only working when you are in the "window" of RPMs to add to the safety of your setup. If you miss shift the nitrous would cut out which would stop any damage to the engine of excess torque coming in at the wrong time.
2.) Controller
or otherwise known as progressive controller, this allows the nitrous to be injected progressively so that you dont just dump all that torque into the engine, again misshift and release at lower rpm it would reduce the damage and pressure on the engine.
So like anything you put too much turbo on at low rev will wear or damage the engine or the turbo for that matter.
Nitrous has a bad name and its because of either chinese whispers or made up stuff of people who say I had nitrous and i blew up my car with me in it... erm okay, or they have bad experience with it because it was simple not installed properly.
Hope that helps mate