This article has a talk page!This page details bugs of Tetris (NES, Nintendo).
Killscreens#
There are two "killscreens" in this version of Tetris, though only one of them is an actual bug. Both are also theoretically defeatable, unlike the killscreens of, for example, the arcade Donkey Kong, but the community still refers to them as "killscreens".
Intended Killscreen#
The first killscreen occurs at Level 29 and is an intentional killscreen: tetrominoes fall one tile per frame, the maximum possible speed in the game's engine. This is too fast for most normal players to complete any more lines, especially since the "auto-fire" piece movement achieved by holding the directional buttons is not fast enough. However, with the "rolling" technique discovered in 2019, players can sustainably achieve movements faster than auto-fire and lines can be completed at killscreen speed, allowing the player to continue on past it.
Unintended Killscreen#
The second killscreen occurs at Level 155 and above. Depending on what actions are taken within a level from this point until the level overflows, score calculations can be interrupted by VBI (vertical-blank interrupt). Since the routine run when VBI fires clobbers the scoring routine's RAM space, this causes the game to crash.
Blue Scuti was the first person documented to have performed this glitch without external tools, a feat which got him fame and notoriety.
Palette Glitch#
Normally, the game cycles through 10 different color palettes for the tetrominoes - every 10th level therefore has the same palette. However, at Level 138 and above, the function used to select a palette overflows, and glitch palettes are generated. This is referred to as the "glitched colors".
Around Level 222, the function overflows again, but the resulting palettes are clones of the ones seen beforehand.
Ultra-Long Level Glitch#
The ultra-long level glitch occurs in every game of Tetris that reaches a high enough level. However, this is not possible in normal play due to the killscreens described above.
Normally, the level increases once 10 lines are cleared, but due to a bug, one level - the "ultra-long level" - ends up continuing until 800 lines are cleared. This ends up accounting for an entire quarter of the lines needed to make the level counter overflow.
What number of level the ultra-long level is on differs depending on what level the player starts at, but generally occurs around the time of the second palette glitch overflow.