Cross-Thread error but no threads!?

Question

Hey,
I have a UI with some buttons.  When you click a button it calls a webmethod and updates some information in my model.  Now.... i can click on this a number of times and its fine.... then sometimes for no reason at all i get a  "cross thread" error on my user control.  It appears at a random time.  I am not using threads!!  The only thing i can think of is that something is threading without me knowing?  I have a Ribbon bar and some buttons and that is it.  Im not even trying to update the UI once the button is pressed!  (only some info in the background).
anyone ever had anything like this?