He allowed a speculative long-range shot from James Beattie to spill out of his hands and roll tantalisingly over the line.
By this stage United were looking a well beaten side - outplayed in midfield by a bargain basement Blackpool squad compared to the Blades side littered with ex-Premier League stars.
United played some of their best football in the early stages with Jon Stead producing some lovely touches.
In the 12th minute a well-judged Stead cross was headed home by Beattie who was left unmarked at the far post.
Rachubka had to pull off a brilliant save to stop a sliced clearance from Shaun Barker creeping into his own net and at the other end Paddy Kenny leapt backwards to keep out Gary Naysmith's badly judged attempted clearance.
Blackpool improved as the game went on with David Fox dictating from the middle of the park and Wes Hoolahan causing all sorts of problems down the flanks.
Just before the break Blackpool equalised when Andy Morrell was hauled down just outside the box.
Stephen Crainey produced a lovely free kick which left Kenny wrong-footed as it curled into the bottom corner.
Blackpool should have had the game sewn up midway through the second half when Gary Taylor-Fletcher and Hoolahan failed to test Kenny when clean through.
Blackpool went ahead in the 87th minute thanks to a lovely build up from Hoolahan who eventually found substitute Andy Welsh whose cross was headed home by Ben Burgess.