If you are using SQL Server 2005 or 2008 Analysis Services, and have been keeping up to date with hotfixes cumulative updates, you may have been adversely affected by a previous round (and your users might not have even noticed).  Essentially you can get incorrect results from certain MDX queries, due to a side effect of a fix first introduced in the following updates:

• SQL Server 2005 SP2 CU #12
• SQL Server 2005 SP3 CU #3
• SQL Server 2008 RTM CU #3
• SQL Server 2008 SP1

You can review KB #975783 for further information about the problem, as well as the fix and/or workaround (depending on which branch(es) you are currently running).

Note that if you are running SQL Server 2005 SP3, you can resolve this issue by simply applying the update found in KB #974648 (9.00.4226).  The other three currently supported branches (2005 SP2, 2008 RTM, 2008 SP1 – and yes this includes those that have applied the updated CU #4 hotfix from KB #976761, buuild 10.0.2740) will have to apply the workaround mentioned in KB #975783 until a proper fix is supplied for those branches.  Note that the workarounds can lead to degraded performance, since they turn off partition indexing.

Incorrect results are always bad.  Any query that can yield incorrect results could, feasibly, be used to drive the reports that serve as a company's billing system.  So this type of bug should always raise a red flag if your invoices are data-driven.