I'm just wondering what you think, and can't fit the question into a Twitter message.
Say I have a function which takes a string, and returns the numeric represent of that string in the context of a set list of values. EG: the function is dayOfWeekAsInt() takes "Monday" and returns 2 (Monday being the second day of the week). What should the function do if it receives invalid input, say "Moonday"?
I'd err towards raising an exception, but another option could be to return 0. On one hand "Moonday" is not a valid input, so an exception is good response. On the other hand "Moonday" has no index in the context of the series Sunday-Saturday, so I can see a case for zero too.