In Duel Monsters 1 sometimes the autosave stops working. This prevents a lot of people from unlocking Yami Yugi. It is already known that Tremendous Fire can cause this bug but the only way to obtain that is by using the enhancement/cheat to give you all cards, which not everybody does. But many people who DON'T use tremendous fire are also experiencing this issue. Sparks and Hinotoma are similar but don't cause issue.

The REAL CAUSE for autosave breaking in most (if not all) cases is performing any fusion that results in Gaia Fierce Knight. It doesn't matter whether GFK attacks, defends or stands there menacingly and does nothing; simply the act of fusing Fiend Sword with any of the 3 cards that produce GFK (Swamp Battleguard, Flame Swordsman, Judge Man) somehow glitches the autosave system from that point onward. It doesn't matter which fusion material is on the field and which is in your hand. Replacing GFK with any card doesn't fix it. Even fusing GFK with Curse of Dragon into Gaia the Dragon Champ before the end of the game doesn't fix it. I haven't tested whether it dying in battle will fix it but I highly doubt it.

I must emphasise that ONLY FUSING to obtain Gaia Fierce Knight causes the autosave to bug out. Fortunately if you fight Yugi 100 times and get the GFK card as a drop it is completely safe to use. You can look at its stats, place it, attack, defend, fuse it into Dragon Champ and the autosave will continue to function as normal. So while GFK sadly can't be used via fusion in the early to mid-game if you grind Yugi then he is still useable late-game.

I haven't performed every fusion in the game but so far Gaia Fierce Knight is the only one I've used that breaks the autosave.

Some fusions that I know are safe:
- Flame Swordsman
- Judge Man
- Gaia the Dragon Champ
- Soldier of Stone/Giant Rock Soldier
- Charubin the Fire
- Darkfire Dragon
- B. Skull Dragon
- Dissolverock
- Dragoness the Wick/Dragoness Wicked

If any of you find any more fusions that break the game, post them here. An easy way to test for them is to battle anybody, do whatever fusion you want, win, then check your stats and make note of who you just battled. Quit the game and reload and check your stats again; if they're the same as before you quit then it's all okay. If your stats went down by 1 then that fusion is broken. Again I've only had this happen with GFK but there may be more.


TL:DR: DO NOT FUSE TWO CARDS TOGETHER TO MAKE GAIA FIERCE KNIGHT!! If you do then autosave will break and you will be unable to unlock Yami Yugi. However obtaining Gaia Fierce Knight from beating Yugi 100 times IS SAFE, so you can use that card in any way you please without issue.