Using Defaults Correctly

Defaults in computer programs I think can lead to precarious situations in some cases. For example, consider this prompt:

Launch nuclear missiles? Y/N Y is default.

God forbid if the user mistakenly entered another key other than 'N' when he meant to press the N key. This is exactly where defaults can lead to big trouble.

The example I showed I admit is a little bit ridiculous. In the first place the default would have been on the 'N.' In the second place there might not even be a default.

A more likely prompt is one where the computer asks the user if (s)he wants to delete a file and the default is on the 'Y' or if (s)he is sure (s)he wants to delete a file if the user wants to delete a file. In either case if the user enters a 'Y' both effects are irreversible or very hard to reverse. This I think is where defaults can be dangerous in those cases. Either have the default on the 'N' or eliminate the default entirely or to be more safe make the user spell out the words 'Yes' and 'No' since there is a less chance that (s)he will hit those letters in exactly that combination that to hit a single character.