Why did some of my reports stop working after an upgrade?

Environment:

All sites, Reports

Issue: 

A report that ran fine before an upgrade is now showing old data or unexpected data. 

Resolution:

There are a number of settings in reports that can be automatically enacted by the system which can cause a report to behave differently than its designer might expect. For example, a report will be automatically set to Deferred Execution if it takes longer than 60 seconds to run. The option Allow server to change the data source as necessary -if enabled- can cause a report run on the replication database rather than on the operational database.

These kind of events can happen at any time. However, during an upgrade there is a lot of activity in the various databases that a site is composed of. In some cases this activity will trigger one of the above options, causing unexpected behavior in a report. Note that there is nothing special about the upgrade "causing" these behaviors; they are normal behaviors which just happen to be triggered by the upgrade activity.