This page details bugs of Nibbles (DOS).
Speed never increases
When the game is launched, the player is asked whether the game's speed should increase during play, however this never happens due to a bug. The player is asked to answer yes or no before the game starts, in this block of code:
DO LOCATE 15, 56: PRINT SPACE$(25); LOCATE 15, 15 INPUT "Increase game speed during play (Y or N)"; diff$ diff$ = UCASE$(diff$) LOOP UNTIL diff$ = "Y" OR diff$ = "N"
However later in the game, instead of checking for a Y or N, the game mistakenly checks for a P to see whether the speed should be increased or not.
IF diff$ = "P" THEN speed = speed - 10: curSpeed = speed
Since the player could never enter a P due to the above loop condition, the game's speed will never increase during play, regardless of whether the player requested it or not.