I encountered a few bugs playing quest mode fantasy.
#1 : The Unlimited tournament deck (or maybe starter deck) is being sold for 995 even after I update the card price list.
#2 : After I buy the Unlimited tournament deck (or maybe starter deck) it is still available to be purchased again. And again, and again. I can make unlimited money/mox with this since the cards inside sell for more then 995.
#3 : I was dueling with ante but didn't want to lose cards so I was making a copy of the file.dat in the /res/quest/data folder. I was just copying it to the same place as he was. So I had a "my_quest.dat" and a "my_quest - Copy.dat" in the /res/quest/data folder. I stumbled quite a few times (like 5-6 times) on a bug that rolled back my quest. I remember clearly on a time when I got a black lotus in a pack, went to make a copy of my file, played a few games, then the cards were missing from both my current game and the copy I made.
Once I stopped going through this loophole I stopped getting this bug.
OK you can judge me now for cheating

I know your first impression is : He messed up the file switching and deleted his current quest. After the first time I encountered the bug that's what I thought and was really careful with the switching.