The only time this happened to me, I just clicked 
where the button should have been and the item 
was added to my cart. At the time (and until now) 
I just assumed the picture for the button hadn't 
loaded or something. I've never seen it happen 
again, but I'm glad to read these posts so I know 
what to try if I see it in the future. ^_^;;

Many Sharp Smiles,
--Drac