Icewind Dale has so many amazing weapons and armor, along with enough XP to go around, that dangerously overpowered characters can be made. Especially as the game lacks enough mages to really counter cheesier strategies. Below is the most OP character I've ever made.
Str (18/00) Dex (21 with items) Con (17) Int (18) Wis (8) Cha (9)
Attacks: 5 per round, 10 with improved haste
Backstab: x5, thief skills all maxed out (with move silently and hide in shadows both at 220)
Resistances: Slashing (30%), Cold (20%), Crushing (5%)
Why OP: The game has absolutely 0 counters to someone who can cast mind blank (24 hour duration immune to charm, maze, feeblemind, confusion, fear, intoxication, hold and petrification), spell immunity, stone skin and improved haste. The game also can't do jack against time stop. You reach the point where casting tenser's transformation to jump from 136->272 HPs just isn't needed.
Claim to fame: Defeating Icasaracht with two time stops, improved haste, and 2x long swords of action+4. It's a mixture of a let down and excitement to see the final boss go down in 6 rounds without losing a single HP.
The only other time I have felt that anything was OP was with a dwarven defender. Hard difficulty with the extra xp allowed. Reached lvl 20, which got him the 20% physical resistance. The glory of suffering +6 pushed that up to 30%. Great Shield made all physical resistances 45%. Clasp of Bron's Cloak turned it into 50%. And defensive stance made all physical resistances 100%.
Completely OP. Enemies do double damage? Well they now do normal damage against the party tank. And if there is ever a fight that might be deadly, just drop defensive stance and he becomes immune to everything that the game could throw at him.
Yes, it was stupidly easy as the enemies weren't smart enough to walk around the dwarf to attack his much more squishy friends.
So those are my two forays into 'damn...' territory. Anyone else want to share?