Doors

Doors are contraptions made of metal, and can only be opened if you hit the blue button (or both if there are two. The door will not open unless both are hit.).

In the introductory stage, there are blue arrows pointing to the switch.

The buttons are reused in the Local Multiplayer Modes for one glass pane.

Buttons
Each door has 0, 1 or 2 buttons. Only hitting all of them will open the door. The buttons are also used in the glass pane with buttons (exclusive to Local Multiplayer).

Outline
In the Tunnel to Endless Mode (first opening in all versions, and second opening as well before version 1.2.0), as well as the Disappearing Room Glitch, and opened-up doors, the doors will have an outline. It is transparent - despite it existing, balls will go through it.

Trivia
The Double-Doors Glitch is caused by the fact that the door at the end of stage 4 of Checkpoint 11 (Mayhem Mode) is not actually in the end, but partly inside the boss stage (invisible line between Stage 4 and boss). So after opening one door, you need to open another door (partly inside the stage).
 * Smash Hit - Official Trailer starts and ends with a door opening.
 * 13 Things You'll Only Understand If You Play Smash Hit has some points on the doors, e.g. Even though that zen is quickly compromised by the almost physical pain you feel when you crash [into a door]. and when you successfully open a portal [door]:
 * 13 Things You'll Only Understand If You Play Smash Hit has some points on the doors, e.g. Even though that zen is quickly compromised by the almost physical pain you feel when you crash [into a door]. and when you successfully open a portal [door]: