=== quote from QB 2006 updates page ==New Information: We have identified an issue that is affecting customers who after updating to QuickBooks 2006 Release 5 attempt to open a data file while two or more users are logged in. In this multi-user situation, the customer may experience a series of delays where QuickBooks appears unresponsive. If the customer aborts the program at this point, the next time the data file is opened the customer will receive a message stating "This data file cannot be used because it is not completely updated. Please restore your backup."
It is important to note that while this situation prevents the customer from accessing QuickBooks, it causes no damage to the customer's financial data.
We have stopped making Release 5 available while we work to fix this problem.
=== end quote == However, further down on the page you can download the R5 patch. So either it is fixed now or the above is seen as sufficient warning.