Reapplying items from your inventory is horribly bugged. The item will disappear and be replaced in the inventory by the item it replaced. But the gear list and stats of the character will still use the old item, not the new one.
Any time you swap floors your set choice of who to give gear goes back to default. Meaning that it's very likely that between you using the drop down to select who to give the gear to, and clicking the button to assign the gear. You'll have swapped floors causing the gear to go to your useless Paladin who is only present to cast +35% attack speed to your 3 DPS every 33 seconds.
If you move the DPS meter when in a smaller window. Then swap over to a fullscreen window. It's possible for the DPS meter to end up behind the gold "button". Thus making it impossible to ever move the DPS meter again, it's now forever stuck there since the 4-dots you have to click to move it is now behind the gold element. Edit: On relaunching the game the DPS meter is now heading north for winter, apparently. The part you need to click to move it is now off screen at the top, never to be seen again.
As mentioned by someone else, the cursor in the skill window is cursed, you basically have to guess where to click. Sometimes the "panning" function also gets stuck on and you end up having to click the background a few times to make it stop panning, to allow you to actually click anything at all.
Paladin's shield is bugged. It uses mana twice, making it effectively cost double. Doesn't apply double shields though. Actually the shielding it applies is super random. Anything from around 8% to 30%, despite stating 15%. 30% makes sense if it can crit. But why on earth it's giving less than 10% is beyond me.
Removing my Pet from my Paladin and upgrading it, then reapplying it to my Paladin made it so the Paladin is not getting the stats from it, it doesn't appear in the Pet tab for my Paladin, but it is applied to my Paladin according to the sidebar.
UI Bug: The loot icon is 1 pixel too big and overlaps the surrounding. Meaning that if you place the cursor on the outermost pixel, the "mouseover" highlight starts strobing as the game thinks your cursor both is, and isn't over the button. Edit: Seems to be true for all buttons actually.
It's possible for your characters to die while still having HP. I routinely have my Paladin be dead and have 2 hp at the same time. Once a new room loads it seems to be a bit random if this means he is now actually alive with 2 hp, or just instantly actually dead. Both have happened IIRC.
Game does not remember loot filter and popup settings between sessions (web) also for QoL being able to set the end of floor popup to 0s to skip it entirely would be nice. Since it's annoying af when you're doing stuff in the side bar and the pointless window pops up and goes "haha, you can't do any inputs for 1 second".
From more of a balance perspective, tanks are completely useless, as are healers. Drain tanking with life steal is infinitely better in every single way. So you just want to run 3x DPS and then Paladin filler to use Holy Fire. The heals and the extra tankiness of the other characters are simply so horribly underpowered that they serve no purpose.
If you're going to have loot filters, and this game needs loot filters, don't make them level/rarity based. It's completely pointless. Give us a per character list of stats that we can order in a priority list at least, or give numerical weights to ideally. Tick boxes like "ignore amulets without mana regen" would also be great. Or, alternatively, since amulets without mana regen are 100% useless. Maybe make it so amulets uncommon and above simply cannot generate without mana regen, same with caster weapons.
Lastly, this might be by design, but currently Vaults are just way too good. It makes grinding the actual dungeons feel completely pointless, other than to just get experience and, I suppose, more vault keys. The likelihood of getting an item that's better than vault items in a dungeon, is so low that it just feels like a waste of time to even look at gear from them for significant portions of the game. Especially in the 10-20 level range. I'm currently in the 20-30 range and dungeons are starting to drop useful loot again. But for a very long time I had 100% vault gear.