Suffocation

Suffocation is when a player is unable to breathe due to his upper half being inside of a block. This exploit was added after Notch discovered players were using sand and gravel to locate caves. Notch soon added gravel inside caves. Due to sand and gravel falling, it can easily cover the player. Riding a pig with a saddle will suffocate you in 2 block tall areas, as you stand one block taller than normal when doing so.

In Survival and Creative, it just blocks your vision; in Indev, you will be able to see through dirt and stone, able to find caves and other hollowed carvings.

After the July 9th update, you no longer suffocate when riding in a minecart. You can, however, suffocate if you get out of the minecart while inside a two-block-high tunnel, as you will be standing on top of the minecart and your head will be inside the tunnel ceiling. The same applies when disembarking a boat.

This video demonstrates a player suffocating. By changing your view into the third person it is possible to see through the blocks.