The bees have built up comb linking the brood box to the super above it - with some exciting results for an Incompetent Beekeeper!
When I lift the top box off (the 'super'), some of the frames in the brood box below are stuck to it. The first time I lifted the super without realising this, the lower frames broke off when I had the super about a foot in the air. The frames that I had unwittingly dragged out of the brood box dropped back down into the brood box with a clunk, upsetting the bees more than somewhat.
This photo is a shot of the top of the brood box, covered in very annoyed bees, honey, broken comb and brood.
Live and fail to learn, that's the Incompetent Beekeeper motto.