It's going to be updated very often. Keep an eye on it.

If you want to suggest something, feel free to do so.
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Fix Golem crash bug. It happens when golem is killed by a monster.

Golem stats should be better balanced.

Fix duping bug.

Replace obsolete Anvil of the Ages with a new base item.

Create new animation for Assassin.

Fix Flash damage from monsters (increase a bit).

Flame Wave should be made useful.

[fixed] Elemental should be made useful.

[fixed] Make bosses' HP viewable.
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Extra info about generic bugs of Hellfire, that weren't fixed in it, but got into Diablo 1.07.

This info is available in 'Update.txt' file in Diablo folder (if you have Diablo v1.09 installed)

CHANGES FROM 1.05 TO 1.07

Serious bugs (crashes or freezes)

A. Fixed crashes associated with being attacked or killed by a
Black Death (Zombie class). This includes the crash upon
resurrecting after being killed by a Black Death.
This is useless, because monsters don't steal HP in TheHell
B. Fixed the freeze upon touching a Hidden Shrine while equipped
with only indestructible items.
Also useless, because Hidden shrine doesn't exist in TH. However, I recall crash reports about touching it when it was enabled (that only proves this bug to exist in Hellfire)

Medium bugs (affecting gameplay)

C. Prevented players from becoming stuck inside a wall when Town
Portalling down into a dungeon.
Almost every player who spent in coop games more than several hours, can acknowledge that. It's there.
D. Fixed bug that made some Hell difficulty monsters too easy to hit.
Beware.
I have successfully fixed that.
E. Fixed the Mana Shield bugs that could make a player invulnerable
and/or invisible to other players.
No idea what it is, but if it was fixed in Diablo 1.07 then TH is very likely to still have this bug.
F. Fixed the bug that prevented Rogues from properly disarming traps
on levels 13 through 15.
Don't know about this. But I'll have to check that.
G. Fixed bug that rarely transformed unique items (like the Optic
Amulet) into some other (usually mundane) item when saving the
game and then starting a new game.
Something tells me that it's this bug that causes quest rewards to morph on restart.
H. Prevented repeated castings (128 or more) of Mana Shield from
disabling the casting of any more spells.
It's there and it's linked to 2 (or more) MS + Fury gamecrash issue that gives error at :00404673
I. Fixed bugs that prevented items with the suffixes Thieves,
Speed/Haste, Balance/Stability/Harmony, and Piercing/Bashing/
Puncturing from working properly.
This was fixed in Hellfire 1.01 patch AFAIK.
J. Fixed the bug that caused Mana Shield to become less effective
with increasing spell level.
Erm, I'll have to check that :)
K. Fixed the bug that caused excessive damage when a Mana Shield
expired due to an attack.
Fixed
L. Prevented bows with fire-hit damage from interfering with the
behavior of Gargoyle class monsters.
No idea what it is
M. Allowed right-clicking (casting/drinking) belt items in positions
5 through 8, while the Spellbook is open.
It's present in Hellfire, TheHell and all other Hellfire mods
N. Fixed the bug that froze Lazarus (and made him invulnerable to
arrows and spells) after player had entered his lair, then left
(or died), and finally returned.
This bug exists in Multiplayer. If go to town with Lazarus activated, and then return, he'll be in immune state until you walk up to him and activate. He can be fought after such activation, though, and will do what he must, including the dying :)

Minor bugs (graphics or sound)

O. When in effect, the Mana Shield graphic appears to all players.
I can remember some bugs with MS ball disappearing on top of the Mage's head when confronted by Defiler's pack. Apart from that, never seen that.
P. The casting of a Nova spell is now visible to all nearby players..
Don't know about this one
Q. Adria's books appear white now only if a player can read them.
True. This bug is definetely there.
R. Equipped shields always appear properly to all players.
No idea what they're talking about
S. Nightmare and Hell difficulty monsters now display the correct
number of hit-points (HP).
Fixed
T. In single player, the sound is no longer shut off by loading a
saved game while Diablo is in his death throes.
Not needed, because Diablo's death doesn't trigger game end.
U. You may now change the name of a character to an existing
Battle.net account.
Useless
V. Fixed the incorrect display of special Battle.net chatroom
icons.
Useless because Hellfire doesn't support Battle.net
X. Fixed renaming character to existing character bug.
No idea what it is

4. CHANGES FROM 1.04 TO 1.05

A. Battle.net accounts with unique names are now implemented.
On Battle.net, you can claim a unique name for your character
and protect that name with a password.
Useless
B. Added ability to /whisper, /squelch, and /unsquelch in and out
of Battle.net games.
Useless
C. Fixed potential memory corruption bug that occurred when
casting town portal.
Could be connected to Catacomb levels morphing. Not sure though
D. Added data protection to direct connect networking module.
No idea
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Fix the bug (reported by Koven) about dead ressurection in town after dying to a burning cross. Also, check 2007-12-16, 3-56, Single-player Burning Cross Gameload Crash.txt to see if those two bugs connected.

Fix: A book can be read even if the max SLVL is reached instead of being red (by Koven)

Fix: Whenever you cancel an action from potion or relict, you lose it (by Poseidon)

Introduce spells with physical damage! (a simple but genius idea by Oli). Probably, modify Flame Wave or Elemental

Check the bug when mana pots give no increase of mana if Corruption item is on.

Warlord has same bug as Lazarus in MP?