Some hyperlinks open in separate windows because they depend on external viewers to display the file contents. This is generally true for movies and for some graphics. To alert you that this is the case, NCSA Mosaic displays the message ``Spawning external viewer'' in the information line at the bottom of the window.
If NCSA Mosaic attempts to spawn an external viewer that is not installed, an error message may be displayed on the console (xterm) from which NCSA Mosaic was executed. NCSA Mosaic may ask you to enter a filename to save the current ``unidentifiable'' document.