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Yang's HP bug is a bug in Final Fantasy IV that stops Yang's HP growth after passing a certain level. This was fixed for the European version of Final Fantasy IV Advance and the re-release of the Japanese Game Boy Advance version. The bug wasn't likely noticed on older versions of Final Fantasy IV since Yang never rejoins the party after the events in the Tower of Babil, and it is unlikely the player will ever level the party high enough to witness the bug before he drops out of the party.
The bug stops Yang's HP from increasing past level 60, making Yang unable to reach 9999 HP. It seems the information related to his HP gain in his level up data was neglected to have been given further increments past level 60.