Jadefire satyrs seem to lack their abilities. Despite being listed in the directory, I can't find them in my copy of Bestiary. Valkyrie Database (which I tend to consider fairly reliable) displays all of them with Jadefire ability, same as Wowhead for retail. Going by these sources, Jadefire Felsworn is also missing Shadow Bolt and Cripple, Rogues Backstab and Gouge, Hellcallers Fireball and Hellfire, Betrayers Cleave and Rend. While these aren't perfectly reliable sources for TBC, being obvious caster mobs, Felsworn and Hellcallers are clearly missing something and those abilities seem really plausible.
Xavathras lacks Enrage and Rend
Prince Xavalis seems to lack Jadefire and Immolate
Grazle doesn't have a response to his dialogue option.
Overlord Ror seems to be missing his Terrifying Roar and Maul abilities.
Withered Protectors seem to be missing Wither Strike. In addition, there only seems to be one spawn, which seems weird. I recall there being at least a few of them there while doing "The Wrong Stuff".
Irontree Stompers seem to be lacking their War Stomp ability
Toxic Horrors seem to be missing their Deadly Poison and Localized Toxin abilities
Warpwood Shredders seem to be missing their Thrash and Sunder Armour abilities, Moss Flayers seem to be missing Entangling Roots
Captured Arko'narin, NPC involved in an escort quest, is attacked by nearby mobs. In addition, you can open her cage, even without the key. In addition, her dialogue shouldchange depending on what part of the quest you are at.
Deadwood Furbolgs seem to lack some of their abilities (a lot of them don't seem to use Curse of the Deadwood, Avengers seem to lack Enrage, Shamans seem to lack Lightning Bolt and Chain Lightning)
Quests
As you bring Kitten Carrier to moonwell in [Corrupted Sabers], it doesn't turn fel, rendering the quest impossible to complete.
Despite having Cenarion Beacon with me, I seem unable to get Tainted Skins for Salve via Skinning. I haven't tried, but I'd wager it's the same thing with the herbalism version.
A potentially exploitable detail! The repeatable "Salve via Foobar" quests continue giving experience in their repeatable versions.
Cleansing the corrupt flowers doesn't work, nothing happens as you hand in the quest.
Takes place in Barren, but as you hand in Seeking Spiritual Aid, the quest giver suddenly teleports to water (and then walks back to his original location). In addition, the water spirit says nothing. The missing dialogue (and emotes?) are detailed in the Wowwiki article.
Rescue from Jaedenar quest autocompletes. Wowwiki describes how the event should work in some detail, although I'm afraid it misses the dialogue and other such details.
The quest category is Ironforge, but I'm reporting these here because the first part takes place in Felwood. The quest giver Laris Geardawdle seems to have incorrect dialogue. He seems to have two alternative topics (he switches between them when talked to multiple times) but "look at these samples" one is clearly related to the quests while the one about artifacts and night elf heritage is what he should say before the quests. In addition, he has 3 discussion topics that do nothing, also something that clearly shouldn't be there if you aren't on the quest (second part at that, probably). Curiously, this very vanilla-esque quest has remained in the game intact. I don't have retail sub but it should be easy to check what those dialogue options contain and when exactly they're shown. In addition, as you turn in the first part of the quest, there's a brief RP event. Finally, at least in WotLK there was a bug of being able to take the samples from the wrong ooze (including live ones) without them ever despawning. I can't find proof of the bug existing in TBC, however, and it's of course debatable if apparent bugs like this one should be replicated.
In the follow-up any slime at all seems to work, not just the Un'Goro ones. Wowhead mentions ability to use Naxxramas Construct Quarter oozes and Vile Oozes in Felwood, but any oozes at all seem to work, including the corrupt oozes that certainly shouldn't if you think about the logic of the quest. On closer inspection, the same seems to apply for the first part as well.
Purified! autocompletes. Wowhead comments give some indication about how it's ought to work but I couldn't find a proper video or other complete information.
Flute of the Ancients can be used as many times as you like without cooldown, summoning Arei an infinite number of times if you'd like to. According to the comments, it should also summon other mobs like treants from time to time. In addition, the escort quest Ancient Spirit that you get from Arei autocompletes. Wowhead and Wowpedia have fairly complete information how the quest is supposed to function.
World
There are cleansed herbs around, without the need for doing the cleansing quest. Moreover, the objects don't despawn. Moreover, the herb doesn't disappear as you harvest them, making them an infinite source of free buff items.
Items
Timbermaw Ally spawned by Defender of the Timbermaw trinket doesn't use its abilities. Wowhead describes the functionality of the trinket.
Mobs
Quests
World
Items