Perhaps the most famous of all the bugs in Jet Set Willy, the Attic bug is responsible for certain rooms becoming impossible to navigate after Willy has visited
The Attic, thus making it impossible to complete the game.
The root cause of the bug is a misplaced arrow, defined by the entity specification at
59900. The second byte of the entity specification is an index into the table of screen buffer addresses at
33280, and determines the arrow's y-coordinate; the index should be an even number, but the value of the second byte is 213, which means the arrow-drawing routine at
37310 looks up the word at 33493 (33280+213) and finds 41066, way outside the actual screen buffer at 24576-28671. As a result, the misplaced arrow is 'drawn' at the following addresses:
which sets the pixel y-coordinate of the arrow to 41.