When I was at the dock yesterday grabbing some quick photos of the birds, after climbing up on the actual dock I went to focus on a bird sitting on one of the pylons but when I hit my BBF button the entire viewfinder remained blurry. I panicked thinking the used Canon 70-300mm IS II USM I had purchased only days before had crapped out on me. However the AF seemed to be moving at least, so I checked the diopter. Nope, nothing wrong there. Then I realized the vf eyepiece was dislodged, and sure enough that's all it was! *sigh of relief*
Figured I'd share my moment of stupidity in case someone else runs into this problem, as it happens easily when you have to squeeze your camera body into a bag.
-Joe