error of "Dump dcd of non-matching # of atoms"

I’ve got the error message of “Dump dcd of non-matching # of atoms”.
It happens when one parameter in force field is changed.

In what case, the number of total atoms changes during the simulation?
It is the ensemble of constant N and hence, N should be the same during the whole simulation.
I think it may be concerned to the neighbor list radius and how often it is updated. It is right?
Generally, what factors cause this error and what I should do for it?

Thanks for your answer in advance.