Opposed to white-box / API testing, which is covered here
There are many interviews, both for developers and qa, where you are asked to test smth out. A classic is to test the save dialog from notepad.
Here is a list of tests I can come up with, categorized and prioritized:
[functional, high] - dialog performs its use cases - comes up when the user does a "save as", the buttons on the left work, the file is saved with the given name and encoding, top buttons work, cancel button works.
[functional, medium] - save to a Read Only medium - does a fair error pop up?
[functional, medium] - save an empty file
[functional, low] - save a file as .txt but delete the extension from the name
[functional, high] - make sure the dialog is modal
[functional, high] - make sure the dir navigation & dir features work
[functional, high] - context menu options work
[load, low] - navigate to a directory where there are a very large (aproaching the system limit) number of .txt files; save the file there
[load, low] - navigate to a directory with a large number of sub-directories. save the file there.
[stress, low] - navigate to a directory with a maximum number of files allowed and try to save the file
[stress, low] - saturate the system with file handles (to simulate apps that have very rich interfaces or handle leaks, like Yahoo Messenger); does the dialog pop ?
[stress, medium] - try to save a files when there isn't space on the disk.
[stress, low] - check how deep the "navigate to last folder" button goes with this
[stress, low] - navigate to a directory with a very large number of directories
[performance, low] - measure the time needed to display the dialog
[performance, low] - measure the resources used by the dialog (memory, window handles, I/O); make sure the resources are dealocated
[accessibility, high] - is the dialog compatible with the accesability tools win provides? (magnifier, narrator); for example that the menu items scale well when choosing large fonts
[accessibility & compatibility, medium] - is the dialog compatible with famous 3rd party programs that provide accessibility features?
[localization, high] - the dialog works on other locales
[localization, high] - random inputs of UTF-8 and ANSII letters as the file name
[localization, medium] - right to left locales
[compatibility] - make sure that the file saved can be opened by other editing tools, no matter what the encoding is
[usability, high] - user is asked if he is sure he wants to overwrite a file
Can you add some more? Or comment on these ones?