I've been playing Hack/Mine on a server with some friends for a few days now and I'd just like to put forward some observations and suggestions. But let me preface this by saying Hack/Mine is hands down, the most badass mod for Minecraft in existence. It practically needs to be it's own game.
That being said, here's a list with a few bugs I've come across:
1 ) Mutant towers - Towers that look like a short tower merged with a tall tower. Most of the time they end up unnavigable. Often there's a wall missing from the short tower. And 90% of the time I encountered one, it was in a snow biome.
2 ) Unopenable chests - Sometimes chests in dungeons spawn 1 block below the dungeon roof, making them unopenable without destroying them.
3 ) Wacky mana bar - The mana bar will sometimes momentarily jump from full to beyond full, extending past it's ui confines and gaining +10 - 30 points. I've narrowed it down to occurring whenever I have a +wisdom item in my item hotbar.
Most of these have probably been made somewhere by someone, or are otherwise pretty obvious. But I figured I'd toss 'em out there anyhow. Most involve NPCs.
1 ) Entity picking - You know, being able to mouse over an item on the ground and see it's stats before deciding to pick it up, instead of just hoovering every damn piece of junk you trounce over. I don't think MC has anything like this in-built, so it may be no minor task to implement it.
2 ) Peaceful town areas - I know Frizzil has mentioned being able to 'cleanse' an area in the future by defeating a boss, but I was thinking something along the lines of town hub areas. You know, the place you go to get the quest to defeat the boss to cleanse the area. Maybe until you cleanse it, the town is unsafe at night. I could even see defense-type quests where you need to protect a village until morning, maybe receiving a reward and some xp for your effort.
3 ) Merchants - A place to unload all the junk you've been lugging around. I guess that would also necessitate some kind of currency.
4 ) Death penalty - Right now I think the death peanlty is the loss of half the durability on your worn equipment. Currently infinite repairs aside, that's a pretty good penalty mechanic. But I'm sure since there'll be some kind of repairing ability on into the future (Currency sink?), and having to revert back to spawn won't be that big a deal once there's waypoints, how about something additional? I was thinking an exponential xp loss. So as you gain lvls, the amount you'd lose grows. Perhaps with early lvls, like 1-10, there'd be no loss. So it's not so harsh on new players.
5 ) Breakables - Who doesn't like a pinata?
And that's pretty much it. Everything else I can think to suggest is obvious, like corridors in dungeons, more dungeon props (spiderwebs, fountains, thrones, etc.), MOAR LEWT!!, some kinda 'town portal', etc, etc. I'll update as necessary, but I'm neglecting studying for my own finals as it is.