Matthew Smith's epic platform game Jet Set Willy was released on the ZX Spectrum in 1984 to a rapturous welcome from the press and public alike. What we didn't know at the time was that it was impossible to complete in its standard form.
Part way through the game, the player would notice strange things happening. Amongst them, the guardians in The Chapel vanished, and you could no longer enter various rooms - including We Must Perform A Quirkafleeg - without dying instantly.
The problem was narrowed down to one of the game's rooms - The Attic. Merely playing that room would trigger the faults.
(Other less famous bugs in the game include an invisible, and unreachable object in the First Landing, a killer object on The Conservatory Roof and a wall making the Banyan Tree impassable.)
One of the hazards present in the game is arrows. They are fired across the screen, so you have to jump over them, or hide in holes in the ground. They fly directly left-right or right-left across the full width of the screen.
It turns out that in The Attic, one of the arrows is set to fly outside the area of the screen - below it, in fact. As the arrow flies through the RAM, it trashes around 288 memory locations (see the Video description of ZX Spectrum for the explanation of this number).
Software Projects publicised a patch (or POKE as we called them in the good old days) for the game, which placed the arrow on the screen, making the room a little harder, but along with fixes for the three other bugs, made the game a whole lot easier - i.e. possible.
The fix consists of the single line: