The wrong leader bug is a bug in Final Fantasy VI.
Before starting a new game, the player can let the title screen and subsequent game intro play all the way through past the credits and back to the title screen. The player shouldn't press any buttons to skip the beginning credits. After starting a new game after doing this, and when the player gets back through the intro and credits, the player can skip these if they wish. Biggs and Wedge will be in the top two slots respectively and Terra will be in the fourth slot. The bug here is that when the group reforms and gives the player control, Terra will be the character shown on the field even though Wedge is the leader. Simply going to the sub screen or moving past the town portion of Narshe will fix this.