An error which appears in Autocomplete taxonomy fields
In my case I get this exact error when using Views. It happens when I try to save changes in a given view and I click too quickly to start another action, while the ajax loader of the previous action hasn't gone away. Basically it's moaning that I should wait for one action to finish before starting another. 

In your case, maybe the ajax request isn't fast enough to go and grab an existing match for the taxonomy term(s) you're adding in order to autocomplete it. Do you have enough memory allocated to Drupal in your machine/server?

If you're using the admin overlay, another trail to follow is comment #10 in this issue