During a Windows Phone 7 course a few weeks ago I was asked about limitations in the ApplicationBar for Windows Phone 7 applications. I mentioned my thoughts on the overall design of the ApplicationBar – e.g. the missing possibilities of sharing an ApplicationBar between pages (there are some workarounds!).
The student asked about any limitations in the number of items – to be honest I did not know the precise answer. I quickly discovered that the limitation is 4 items. If you try to add more items then 4 items you will get a warning in design time and XamlParseException at runtime (inner exceptions is “Too many items in list”).