We've come across a problem on our farm where old versions of workflows are unexpectedly activating. 

As one example, we have a core workflow that has been updated frequently over the last few years.  One morning, users suddenly complained about receiving dozens of emails for a single field change.  After investigation, we found in the workflow settings page that all of the "Previous Versions" had suddenly been set to "Allow" instead "No New Instances".  The single field change was firing an instance for each previous version hence all the extra emails.  

Any idea what might have caused this?  It has happened on multiple workflows throughout our site.  Right now it appears isolated only to 2010 workflows that existed before our upgrade to 2013 (although the workflows were fine immediately after the upgrade). 

Hi,

You will typically encounter this issue when a new version of the workflow had been deployed (same assembly with a different public key token due to the change in the strong key file used to sign the assembly).

In these situations, the existing workflow associations made from older versions of the workflow are automatically set to "Allow", meaning that new instances of the workflow for those workflow associations are allowed.

Thanks.

This was a SharePoint designer developed (not Visual Studio) workflow.  What would cause the strong key to change?

This happened to us today,  All the SharePoint designer 2010 workflows were set to allow new instance.  Even previous versions got activated.

Did we ever got any solution for this...