Commander Keen 1-3
The ceiling pogo glitch
When Keen is approaching a tile from below, he can avoid collision and enter the ceiling under certain conditions.
The RNG is seeded from the initial RTC time and only invoked when randomness is required. Having critters that behave randomly on-screen is the only way to advance the RNG. These include pink robots and all Vorticons.
Commander Keen 4-6
Tricks all three games have in common
Impossible Pogo Trick
Tapping ALT and CTRL near-simultaneously while holding Left or Right allows Keen to perform a humongous pogo jump.
Continuous Vertical Pogo
When approaching a static soft platform from below under certain conditions, Keen can land on it without having to wait for gravity to deplete his vertical momentum.
Landing on corners and hitting them in midair can slightly boost Keen forward.
Keen moves forward 7/6 times faster when traversing a downward slope. When traversing a sharply downward slope, he moves forward 1/3 times faster. Conversely, he moves forward 5/6 times slower when traversing an upward slope and 2/3 times slower when traversing a sharply upward slope.
Death flag glitch
Saving and reloading a game causes the "you are dying" flag to be reset while still leaving Keen in the dying animation, allowing him to clip through walls. This is beneficial when done near the vertical edge of a level, as touching one will complete the level. Oracles and swimming gear cannot be touched with this glitch.
Pogo activation and deactivation in midair allows for greater control over jump height. Keen's pogo hitbox is shorter than his jumping one, which can enable him to jump farther and higher in constrained spaces.
Tricks exclusive to Keen 4
Tricks exclusive to Keen 5
Being pushed under certain conditions, including while hanging onto a pole, causes the end of the level to warp you to the game's secret level, Korath III.
This is useful as the exit from Korath III takes you to the segment of the overworld just prior to the second last level of the game (as the exit to the secret level is in that level), meaning, if it's done early enough, you can skip a considerable number of levels.
This glitch only works in v1.4 of the game.
Initial Shockshund positioning and Shikadi Master behaviour can only be manipulated by changing the initial seed through adjusting the initial RTC time.
Volte-Faces seem unmanipulatable.
Tricks exclusive to Keen 6
Bullet collision abuse
In Commander Keen 6, the Neural Stunner's projectiles have the same collision property as the Gik. Because of this, Keen can stand on bullets.
Impossible Bullet Trick
When standing on top of a Neural Stunner projectile until it disappears, the game becomes confused as to where the bottom of Keen's clipping box might lie. When Keen fires a second shot that's in the same vertical column as he is, the game thinks Keen is standing on it and clips him there.
In Keen 6 v1.0, acquired keystones aren't cleared when exiting a level.