I've disassembled the game to find out HOW the bug works, so I'll be editing this post shortly...
How the bug works:
It's stuck in a loop somewhere. It's incrementing a variable (let's call it Y), then checking if it is equal to the max value. Unfortunately, the max value is ZERO, and it's comparing after incrementing. So it's stuck chugging away in the loop way beyond the loop's max value. In normal circumstances, Y will never be greater than 3, but this is a crazy wacko out of bounds loop.
RAM involved:
address 93+ (Contained 55 49 49 49 when I looked, probably 4 bytes large)
address 68+ (Party member numbers, 01 02 03 03 when I looked)
The bug algorithm:
Y=Y+1
If mem[93+Y]&1F == 1 //this is the swamp tile check, fortune telling room has swamp as tile #1
X=mem[68+Y] //normally, X is between 0 and 3, but in this case it's way off!
Deal Swamp Damage to party member #X
Repeat until Y overflows 255 back to zero
Swamp Damage:
hp=mem[71C+X*2] //16 bit value
hp=hp-2
If hp went below zero, set it to zero
mem[71C+X*2]=hp
What happens is that this gets called with some invalid X that came from nowhere, so it messes up your stats. The exact effects are only predictable if you look at the entire Zero Page memory contents.
For example, I took a memory dump, and ran a program to calculate X*2 for each Y where it would deal swamp damage:
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. .. 0E F4 .. .. .. .. .. .. .. .. .. 92 .. ..
.. .. .. AC .. 06 .. .. .. .. .. .. .. .. .. ..
.. .. .. .. .. .. .. .. .. .. 00 .. .. .. .. ..
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. .. .. .. .. .. 16 .. .. .. .. .. .. 30 .. ..
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. .. .. .. .. .. .. .. .. .. .. .. 12 .. .. ..
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. .. 00 .. .. .. 0C .. .. .. .. .. .. .. .. ..
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. .. 00 .. .. 68 .. .. .. .. 4E .. .. .. 00 ..
00 .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. ..
Depending on the value of X*2, it will decrease one of these by 2:
Current HP 0
Current MP 8
Maximum HP 10
Maximum MP 18
Status 20
Experience 29
Name 40
Items 60
Wizard's Battle Magic 80
Wizard's Field Magic 83
Pilgrim's Battle Magic 84
Pilgrim's Field Magic 87
The 68 on the table is key, that modifies the second party member's first item. That value isn't always going to appear on the table, I was just really lucky to get it.
