An index out of bounds error can occur while rendering a report in Local Report Viewer Mode with more than one dataset.
Object reference not set to an instance of an object, an error occurred during rendering of the report.
System.IndexOutOfRangeException: Index was outside the bounds of the array.
This is thread safety issue and occurs more frequently on a multi-CPU computer but can happen on a single CPU computer as well.
The issue is time sensitive you can attempt to open (render) the report again and it often resolves the issue.
Reporting Services, Render, Report Viewer Control, Data Set
SQL Server 2005 CU Bug # 50003629