The day before this one I had noticed a hair going across the viewfinder. It finally drove me crazy enough to get out the hand air blower and blow it out. I didn't see it in the viewfinder so thought it was gone. It wasn't - it had just relocated to a place behind the mirror, in front of the sensor. I didn't realize it until after I had taken a bunch of pictures of Old Faithful that very clearly shows the hair (lower part of the frame here).