An error which appears in Autocomplete taxonomy fields Did someone ever note this annoying D7 taxonomy error? It jumps out when I seem to "pass" a certain amount of terms or characters. The error is as follows: 


  An AJAX HTTP request terminated abnormally. Debugging information
  follows. Path:
  http://pancherim.co.il/taxonomy/autocomplete/field_engines StatusText:
  ResponseText:  ReadyState: 4
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