MailItem: "Can’t open this item. You don’t have appropriate permission ..."

[Sebastian H]

I added a custom field to an individual (received) mail item with the form editor, and when I tried to reopen it, I got the message:
"Can’t open this item. You don’t have appropriate permission to perform this operation."

I wonder who denies me that permission; I own my computer, or so I think. Nowhere did I say that that item should need special permission to be opened. I really wonder how this could have happened; I only edited the properties for a few controls, how in the world can this affect the permission for the whole mail item? (At least one advantage of the fact that the properties dialog is modal is that it is practically impossible to edit the wrong properties, one would think.)

[Sebastian H]

OK, some further playing around with this indicates that the culprit may have been not that item, but a test form I created and (unnecessarily) published. That form contained only a custom field; after I deleted that, the above error message did not reappear.

Is there any way to get rid of such a custom form? The "Design Form" dialog shows "MyTestForm1" under "Personal Forms Library"; but the Delete button is grayed out.