I'm sure most of us have done it at least once ... I had a position where one of my pieces were under direct threat - I was a pawn up, but my opponent attacked my rook with his bishop and threatened to get the pawn back if I was careless. I tried to calculate every possible continuation and got lost in the possibilities ... and before I knew it, I was totally absorbed by the position 4 or 5 moves ahead and completely forgot that my rook was hanging. Keeping a mental eye on the future position instead of the position in front of me, I made a silly move (moved my knight, if I recall) and lost my rook.
Naturally, and quite embarrassingly, my team mates all wanted to know what happened since it looked so good for me at one point. Instead of trying to explain the intricacies of the position, it would be convenient if I could tell my club mates that I lost because of an "X-blunder". Does this type of blunder have a name?