i am using input fields with drupal core autocomplete feature via module "Autocomplete Widgets for Text and Number Fields" (http://drupal.org/project/autocomplete_widgets). I attached a simple textual content (cck) field with autocomplete feature to a form.
If you load the form and immediately after this load the connection to server gets capped (in times of mobile data connections no rare event), a HTML error 0 appears in firefox + IE after every character input into this field.
This error seems to be caused by autocomplete core-implementation according to the module maintainer (see).
The same issue appears for the autocomplete user-name input field of the masquerade module (masquerade block).
For me this is a usibility issue for a website, so i want to report it as a bug. A normal user has no idea, what to do in these situations.