Usually, the result looks like this: □
A kind of empty rectangle. Sometimes it shows the code for that character within the rectangle.
What do yo call this phenomenon?
If there is no name, is there a common term referring to this "rectangle"?
Some Japanese people call this "tofubake" because rectangles look like tofu.
(It is not mojibake. Mojibake happens when character encoding meta-information is lost. It is not the problem here. Using a better font solves this phenomenon, but does not solve mojibake. Selecting a character encoding solves mojibake, but does not solve this phenomenon.)