Minecraft wiki entry for Debug_screen, associated text: F3 + A : Reload all chunks.
F3 + B : Toggle visibility of hitboxes of visible entities. It also shows which direction entities are looking, as well as the entities eye level.
F3 + C : Copies the player's coordinates and rotation in the form of a /tp command.
Holding for 10 seconds instead forces a debug crash. This generates error code -1, which is misinterpreted by launchers as "video driver error"
Ctrl + F3 + C forces a Java crash instead of a regular Minecraft crash. However, with some keyboards, it triggers only with right control (or rarely, vice versa), thus using left control produces a regular crash.[3]