The combo multiplier effect is a bug with scoreV1 which allows the score to count backwards. This is a flaw with the 32-bit signed integer where (in computing) the max integer is 2,147,483,647
. Once that cap is reached, the score will begin to count backwards. ScoreV2 fixes this problem by capping the score at 1 million points, not accounting modifiers.
The combo multiplier effect occurs in osu!, osu!taiko, and osu!catch. This occurs because the listed game modes use the player's current combo as a part of the score calculations. Meaning that a player would get a higher score if they obtained a full combo (FC) than someone who played the same map with a broken combo.