Gnumeric is an excellent spreadsheet application and gets a lot of use in our house. Every now and then, though, you can hear a "!Q#z$%* Gnumeric!" from me or my wife, because we didn't pay attention to cell formatting.
By default, every cell is formatted 'General', which means Gnumeric guesses what type of data you enter in that cell. Gnumeric seems to be particularly fond of dates, and strings that are definitely not dates get interpreted as dates anyway. If I enter 12/3, Gnumeric uses my Australian date format preferences and displays 12/3/2012.