Invalid postback or callback argument - for dropdown postback on page that doesn't have a dropdown postback

I'm seeing this error a lot lately on our site: Invalid postback or callback argument.  Later in the error description, I see this: at System.Web.UI.WebControls.DropDownList.LoadPostData (and ProcessPostData). I'm confused since the page where this is happening does not have an auto-postback dropdownlist control.
Thoughts?
TIA,