don't use a spreadsheet as an address book
Or really any application where strings containing digits and operators are not actual numbers. Or times can be mistaken for dates. Actually, despite using Excel a lot and finding it incredibly useful, I hate the stupid guesses it makes/forces on you when it comes to pasted/loaded data. It's just inappropriate if you need to get out exactly the data you put in, which is a shame, 'cause it could be great for a lot of ETL situations where sizes are not excessive.