Endless Mode

The Endless Mode (also known as Checkpoint Infinity, Checkpoint ∞, Checkpoint ♾️, and Checkpoint Endless) is a level that extends infinitely. The obstacles and soundtracks are all repeated, except for the Brick walls and the second stage's soundtrack (download here). Obstacles are only seen in the black color.

The endless mode is the longest checkpoint, taking 16 minutes (32 seconds for each stage like most others). It has 30 stages in total, if the harder variants are counted.

Ballcounts

 * As of November 29, 2019, 2601:643:180:5070:0:0:0:7499 (with the high score of 36000 in Mayhem Mode) reached the 1st endless mode with 10,210 balls without hacks or cheats in the Mayhem Mode due to the Random Rooms Glitch and the Ball Grinding Method.
 * Normally, a person would only get around 800 balls, playing with around 750 at the start of CP11. Up to 1,000 is possible if rooms generate in your favor.

Stage 1 (a.k.a. Wide)

 * Spinning glass panes
 * Falling Glass Planks
 * Lasers
 * (Stationary) Swinging glass slabs
 * 2-button Door

Stage 2 (a.k.a. Narrow)

 * Brick walls
 * Cylinders
 * Swinging Hammers
 * Glass Canons
 * 1-button Door

Stage 3 (a.k.a. Open)

 * Sliding glass panes
 * Pop Up Glass Sheets
 * Spinning thick glass propellers
 * Square on Strings
 * 1-button Door

Stage 4

 * (repeat Stage 1 again but slightly faster)
 * 2-button Door

Stage 5 (a.k.a. Space)

 * Thick glass bars
 * 1-button Door

Tips and Tricks

 * Stash up on tons of balls here (I got 800). When the stage gets faster, you'll need them to protect against crashing, especially into Cylinders.
 * Your Quadruple Balls are especially good at taking down the thick Glass bars.
 * Boom!.jpg Explosive Ball can be used to its full potential here, taking down Cylinders, Lasers, etc.
 * Use Quadruple/quintuple Balls to take down cylinders, if your aim is perfect. (Usually works only for slow stages)
 * Time Slow can be used in fast (yellow and red) stages, especially taking down Cylinders, Lasers, Crystals (especially Pyramids).
 * Multitasking with balls (Screenshot 20191028-133327).jpg, and Falling Glass Planks, all with the one Quintuple Balls. What a big Smash!]]Multitask with the balls, smashing multiple obstacles with the one set of balls.
 * Infinite Balls are a useful Power Up, found in abundance here. Use it in fast (yellow and red) stages, for taking down enormous amounts of obstacles.