By default, the error messages which do not require the equation to be retyped are suppressed, because they slow down the graphing process, cover up the equation, and are somewhat annoying when you know the graph shouldn't produce any values in some area. (Also, it saves you the effort of specifying the domain for each equation.) If something goes wrong (e.g. the graph doesn't show up on screen when it should and the reason isn't readily apparent), you can turn on the warning messages (using the Warnings function of the Options menu) and redraw it to see what the problem is. You can use the same menu option to turn the messages off later.
See Warning Messages for a complete list of warning error messages.