Missing post data in Internet Explorer (bug?)

I had a really weird bug recently. I had a drop-down user control that
would not fire the onselectedchange event server-side. After nearly
twenty hours of debugging, I finally setup a local proxy (WebScarab,
which rocks by the way) and noticed that IE was deciding not to POST
the key/value pair for the event, but Firefox was.

I double- and triple-checked the XHTML, and the thing is valid. Here is
the post data, in summary:



If I simply moved the user control up the page, the event started
firing server-side and the POST data Content-Length got longer (IE6

Any thoughts? (For security reasons, I cannot publish the full POST
data or XHTML.)