For example, phone numbers should include the country code if you expect foreign users. Units of measurement are another thing you have to watch out for. If you're an American, chances are that your measurements are not done with the metric system, which the rest of the world diligently follows. So keep that in mind when you're describing the size of your 3 foot widget. Another issue is the way dates are written.
Graphical icons can also spell confusion for foreign readers.
However, in Sicily it means something more along the lines of inserting a thumb into one's private area. Another type of symbol to watch out for are religious and mythological ones. Think about it from other people's points of view. Even simple straight forward icons that we wouldn't think twice about being misinterpreted can be confusing. For example, you have a picture of a cute, little dog next to your search engine which Americans commonly interpret as a fetch or retrieve.