During the Prologue cutscene, the narrator says Gorion "interrupted your chores in the middle of the day", yet the game starts at 7AM.
Dreppin always says "Nice day, ain't it?" even though weather in AR2600 is as random as everywhere else, and it can be raining at the time.
Enhanced Edition changed 8 Reputation to not carry Reaction penalties, yet the descriptor still says "Disliked" at 8 Rep.
If you kill Edwin in Nashkel, even without ever talking to him, Dynaheir's "Edwin is dead" speech will override her "I'm being rescued" speech and you get no XP for doing so.
Bandit Scalps can't be put in the Bag of Holding(BAG31)
XP reward from Tenya depends on how you deliver the news to her. If you pick the "They are all dead and will trouble you no more" response, you'll only get 1000XP. If you pick the "Here is the bowl" response you get the full 2500XP.
Several houses in BG South/Docks are hard to get into because the transition point is off-center, but the house with Larriaz in it is the worst; you can get stuck on the doorway just trying to get in.
During the Seven Suns quest for Scar, if you free Jhasso then return to Scar without killing all the Doppelgangers, you get 2000XP and 2000gp for bringing him the news, and after killing all the Doppelgangers he'll still give you the full 6k XP and 6k gold. In other words, doing the quest "piecemeal" is worth 2k XP/gold more than freeing Jhasso and killing all the Doppelgangers on your initial visit to Seven Suns, which is only worth 6k XP/gold.
Delorna, one of Shandalar's daughters in Oberan's Estate, is supposed to summon guards if you demand the ritual components outright. This doesn't happen.
Brielbara disappears at night. Don't know how intended that is. Usually quest givers don't move at all. At least put her in Splurging Sturgeon at night so you can still get the quest.
In Entar Silvershield's estate, on the upper floor, the wall in Skie's room doesn't actually block line of sight. https://i.imgur.com/PDWUm8U.jpg
You can pickpocket Varci Roaringhorn outside of Tremain's house for an extra Shield of the Falling Stars.
There's a trap in High House of Wonders on one of the four containers by the wall that is impossible to disarm, but does nothing.
Cyrdemac's "Ruffian" bodyguards drop Bandit Scalps. Don't think anyone would let these guys into Baldur's Gate.
Chanter and Voice of North/South/West/East are positioned incorrectly in Candlekeep revisited area AR2626 https://i.imgur.com/PU1Z7vq.jpg
In Candlekeep Catacombs, you can pickpocket Arkanis for "unique" Arrows that only stack up to 40. It is possible Dalton(in Durlag's Tower) has them as well, but I forgot to check.
You can pickpocket Ike outside of Durlag's Tower and then again inside(or buy it) and get two Dwarven Wardstones that way.
The altar with Tome of Wisdom in Durlag's Tower thrid floor AR0504 has a trap with 110 Find Traps disarm difficulty, that nonetheless is impossible to disarm no matter your Find Traps score.
Combat Music loops endlessly in Candlekeep Caves AR5506.
Some NPCs that give info about Sarevok gaining power do so even after you expose him in the Ducal Palace; personally I only caught Herschel.
Sea Charts are not tagged as a quest item and can be sold to any merchant that buys scrolls/books.
Western alcove in "Urn Room"( http://forgottenwars.com/bg1/ar0512.htm
) has a different search sound than the others.
On Durlag's Labirynth level 3 AR0513, the wing-flapping sound of Greater Wyverns persists after their death.
Again on this level, the battle with Fission Slime doesn't work - the moment you split it once you get teleported to the Chess area, even as you see it splitting.
On Durlag's Labirynth level 4 AR0514 Clair De'Lain says she has "a bow trained at your neck" even though she clearly is equipped with a sword and a shield(she does actually have a bow in her inventory, but no arrows)
The description of the Cloak of the Shield says it has "unknown" amount of charges, even though in EE you can see the number of charges on the item icon.
Cult Archers which appear in Ulgoth's Beard when you return with the Soultaker Dagger cannot be attacked until THEY attack YOU.
In the basement of the warehouse where you fight Aec'Letec, Tracea Carol is so low level she instantly dies to Cloudkill. Some cult leader she is...
After you kill Karoug and return to Kaishas which turns the entire village Werewolf-hostile, Dradeel re-appears in front of the "Great Hut" even if you already killed him in his house on Isle of Balduran North. (Note: only happens if you give him the Spellbook and THEN kill him)
A number of quests in the game never seems to complete: Rising Tensions with Amn; Kivan's Bandit Hunt; On the Trail of Sarevok; Slythe and Krystin; Tandem in Extremis; The Rise of Sarevok.