Lets say you used Nepalese passport to enter Nepal. On the way out, u have to show US Passport for them to let u pass the immigration counter. Once they look at you US Passport, they will be like,"sir ko ta Visa pani chhaina, arrival stamp pani haneko rainachha, kaile aaunubho, kasari chhirnubho"
You'll say, "Nepali Passport chha ni ta tyo dekhayera chhireko ni." That's when he will call the cops and you will be detained for lots of questioning n fines n whole lot of problems.