A number of countries' citizenship laws state that a person will lose citizenship if they acquire another citizenship voluntarily. These countries include Austria, Azerbaijan, China, India, Indonesia, Japan, Kazakhstan, Nepal, the Netherlands, Norway, Saudi Arabia, South Africa and Montenegro. (Exceptions exist for a few of these countries but they're generally quite tricky.)
However, I'm curious how an adult would involuntarily acquire a citizenship. There are the obvious ways to get citizenships involuntarily, like by birth on the soil of that nation/jus soli, or by birth to parents of that nation/jus sanguinis, but everything I can think of suggests involuntary citizenship only happens at birth and is not possible for adults.
Thus, Is there any way an adult would gain citizenship involuntarily?