Hitting the note with the incorrect key (i.e., cross instead of circle) will give you half the rating's score and will break your combo. The game will also note it by using a different color to display the rating on the bottom corner. During the final results display at the end of a song, both your correct and incorrect hits are counted together, but your incorrect ones will still be reflected on the score.
A combo is when you hit consecutive notes with either a "COOL" or "FINE" rating; hitting any lower than that, hitting an incorrect key or missing a note will break your combo. Starting from 10, you'll get a 50 score bonus for each extra hit in your combo, said bonus increases by 50 every 10 hits and tops at 250. If that doesn't make sense, look at the table.
||Bonus per hit|
|50 and upwards
Therefore, at 35, you get a bonus of 2,400 due to (50x10) + (100x10) + (150x6).
Every song has a section called "Chance Time" (CT) which is usually located at the end of the song or near it. You'll know when you're in CT because black bars will pop-up in the top and bottom of the screen (temporarily covering your score), and the text "Chance Time" will show up at the bottom of the screen. CT starts from the first note you see with a rainbow trail and ends once that effect disappears.
For every combo hit you get during CT, you get a score bonus which starts at 100 and grows up by 100 with every hit. I.e, :
1st hit = +100
2nd hit = +200
3rd hit = +300
It maxes out at 50hits/+5,000, from there on, every extra hit will be worth 5,000.
If you break the combo, the bonus starts building up again from 100, so it's on your best interest to don't miss a single note, or do so in the beginning to lessen the damage.
A 50hit long CT would give you a bonus of around 125,200 if you don't miss a single hit, and that can make a HUGE difference on your final score. Granted not all songs have a CT with that length (some are shorter, some are longer), but still, generally speaking, getting a perfect or near perfect CT is the only way to get a "GREAT" rank in some songs, it's even more important than doing long combos.