Getting the "Lefty Flip" power-up against a human opponent essentially means that they'll miss the next string of notes.
The "Whammy Bar" power-up. Though not as long lasting, it guarantees you're going to miss some notes, or a lot of notes if your opponent hit you at the right spot while playing an expert song ("Through the Fire and Flames" is particularly vicious).
On some of the more difficult passages, getting "Double Notes" pretty much guarantees that a human opponent will bomb the section, as sequences that are normally executed with hammer-ons and pull-offs not only need to be strummed separately, but now you've got chords as well—and because you have twice as many notes, you'll fail twice as fast. See what happens when you get Double Notes on certain passages of "The Devil Went Down to Georgia" on Expert, go on, I dare you.
Whether using Hyper Speed to space out and speed up the notes is a Game Breaker is disputed among fans. Score Hero permits scores achieved with it, and some players, particularly those aware of speed modifiers in other music games like Dance Dance Revolution, see absolutely no problem with Hyper Speed, especially since it's merely a visual enhancement, while others think that it's cheating because it makes the game easier, and is therefore horribly wrong and unfair.
The DJMAX Portable series has the AUTO stat, which allows you to convert a set number of missed notes into MAX 1%s. In DJMAX Portable 2, one of the characters, Dr. Storm, has a stat of AUTO +6, the highest of any character in the game, meaning you can get away with 6 missed notes and still full combo the song.
DJMAX Trilogy has the TECH stat; in DJMAX Portable, it causes your Life Meter to recover faster. But in this game, it takes on a whole new meaning: it expands the timing windows. You can now more easily get perfect "accuracy" on songs, and no one will know you're using it because stat enhancements are not shown on the result screen.
DJMAX Portable Clazziquai Edition introduced the EASY LONG modifier, which allows you to get 100% on the timing for the end of a hold note, as long as you are holding down the button in question when the note ends.
In Rock Band, Overdrive on the drums is activated differently than the other three instruments. When your meter is full enough to deploy it, the game will present a short section where you can bang around on the drums to create your own fill, then hit the green pad to activate Overdrive. However, this section usually replaces an actual drum fill in the song, and the game will keep bringing it up to give you more chances to activate if you don't the first time around. By never activating Overdrive, a drummer can pass a maddeningly difficult song with 100% notes hit (albeit with less points), without actually playing the hardest fills!
The ONLY exceptions to this rule are drum solos, which do NOT have any fills in them. If 6 Were 9 has it's hardest part as the entire second half of the drum solo - well after any drummer's overdrive would've drained. Trying to pass this song without bandmate assistance is hard to say the least.
Fortunately, "No Fail" mode is entirely allowed in Rock Band 3, eliminating the need to do this.
In The Groove and Stepmania have C[onstant] Mods, which force the arrows to scroll at a certain speed (as if the entire chart was the C mod's BPM). This eliminates any BPM gimmicks and stops, making the chart much easier to read. It does disqualify you from high scores, but it doesn't disqualify the player next to you who's playing the same chart.
Also in most dancing games, but especially E舞成名 ("E Dance Famous" in English), where the steps all border each other, you can try "cheesing" a difficult song by repeatedly stepping on all the buttons at once (half right, half left). However, this only helps with passing the song, not getting a good score, and many games penalize excessively early steps by handing out some sort of "bad" rating; for example, stepping too early in DanceDanceRevolution results in a Miss.
Space Channel 5 Part 2 has a code that allows you to sit back and watch as the game plays for you and gets 100% ratings. It's not that easy to get though. You have to either find it online or go through the extra reports and save Mr. Blank.
Cytus has hold notes. In theory, you're supposed to press down on a hold note as the scan line passes over them, then keep it held down until the line finishes crossing over it. This would be the case if hold notes actually had timing windows; as long as the note is pressed down for its duration, it doesn't matter if you've been pressing on that spot for the past 10 seconds or so, you will get a Perfect. This trick can be abused to make certain charts much easier, such as L2 Ver.B on Hard.