Originally Posted by thirdsun

Use of the DH in the AL is optional; however, "the manager must designate a DH prior to the start of the game; failure to do so forfeits the right to use the DH, and the pitcher must then take his turn at bat.

The designated hitter may not play a field position and he may only be replaced by another player not currently in the lineup. However, the designated hitter may change positions to become a position player at any point during the game; if he does so, his team forfeits the role of the designated hitter and the pitcher, another player, or a pinch hitter must bat in the newly-opened spot in the batting order.

The designated hitter could also become the pitcher, in which case the pitcher or a pinch hitter must hit when that spot in the batting order comes up again. Likewise, if a pinch-hitter bats for some other player (such as, hypothetically, the first baseman) and then remains in the game as the pitcher, the team would forfeit the use of the DH for the remainder of the game.

If an AL manager omits to designate a DH, the pitcher can be inserted into the game.

On May 17, 2009, in a game against the Cleveland Indians, pitcher Andy Sonnanstine was included in the starting lineup due to a mistake when Rays manager Joe Maddon entered the wrong lineup card, which had both Evan Longoria and Ben Zobrist playing third base, omitting the DH spot.

The last time a pitcher was in the initial batting order in a game between two AL teams was September 23, 1976, when the White Sox batted Ken Brett eighth against the Twins."

Quoted from Wikipedia