I've seen that too.

There was something in Microsoft site that taled about it but sorry I cannot find the reference. Anyway they didn't had the fix, it was saying that this could happen whith formset with too many forms, or in other word when the form is too much loaded.

I'll tell you what I found for my own problem and you'll see if it apply to yours.

First, it only happend when I was doing this:

Open a form.
Minimize the form
Close the form with the "X" (the form is still minimized)
Open another form or the same one. Bag the error happenned. (all the time).

My first "fix" was to maximized the form before closing.
My last "fix" is to add a wait windows both in the formset unload method and also in the formset release method. I use wait windows with time out. The required time out was tested on our machines but it vary depending on the speed of the machine itself.

IF you need more information, tell me and I'll dig out the exact code for you.

Please let me know if you find any other info on such errors. I'm sure my solution applies only for specific cases and there must be other situation.

Good luck.