You can't just close an issue without providing a solution! Please do not close this issue unless a fix is provided.
Here is a step by step description how to reproduce this bug. Any Browser and any Server will work. I have tried this a lot.
1) set up clean drupal install
2) install cck
3) install filefield 3.0-alpha7
4) goto Home › Administer › Content management › Page
5) add new field: Type "File" and Operations "Fileupload" label and field_label "file"
7) in global settings on the next page set "Number of values: " to "unlimited"
9) goto node/add/page
10) You now see two fields for files: select a file but don't hit upload yet
11) hit "Add another item"
12) JS-alert pops up which says:
An HTTP error 0 occurred.
Hitting "save" displays json code, which should have been evaluated when i had hit the "Add another item" button. Then all input is lost. Very frustrating....
Again: Do not close this issue, unless a reliable fix is provided
All i know that it's related to the /misc/ahah.js which throws this error.