Question
What are some known issues that will have you clear out the tag table?

Answer
Here are some articles related to errors with the Worker Report:

Error, "Cannot insert the value NULL into column 'tag_collection_id', table 'Altiris_Incidents.dbo.tag'"
http://www.symantec.com/business/support/index?page=content&id=TECH12142

Worker Home page fails to open, but gives an exception error
http://www.symantec.com/business/support/index?page=content&id=TECH12713

Here are some articles related to errors with Deleting Incidents/Edit Multiple Incidents:

Query to delete incidents does not display any incidents
http://www.symantec.com/business/support/index?page=content&id=HOWTO4090

A basic query to clear out information for a user in the Tag table:
     
How to clear all settings for a specific user
http://www.symantec.com/business/support/index?page=content&id=HOWTO1003

In using these Articles please make sure that the message you are seeing is similar to the error message talked about in the article.