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.