According to this announcement:
There has been an error discovered in the GBA software Final Fantasy IV Advanced, which went on sale 12.25.2005. When 4 of the conditions below are met, the game will not be able to operate, and save data will possibly become corrupt.
1: When party members total less than 4
2: Activate the "Item" menu
3: Move an character to a blank space
4: Use an item again after moving the character in step 3.
How to avoid:
Don't do the steps above. After step three, if you choose to save, or choose to view the monster album, be sure to close the menu screen before using an item.