While dualing a Warlock and Paladin duo, I have come across a couple quests that flag you for PvP so I thought I should make a post pointing them out so other players can more easily stay within the dualboxing rules and avoid being banned.
Some quests will flag you for PvP when you attack quest mobs, indicated by "(target name - faction)" others will flag you for PvP as long as you have the quest in your log, and will usually have the (PvP) tag in the quest log.
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Ashenvale:
[A] Mage Summoner - lv 25 (Sarilus Foulborne - Undercity)
[A] Forsaken Diseases - lv 29 (Forsaken Asassins, Forsaken Infiltrators and Forsaken Dark Stalkers - Undercity)
-- This can be done without flagging for PvP if you send a pet on passive into the camp to distract while one character picks up the quest item --
[A] The Mortal Strike - lv 31+ (Commander Grushak & Farseer Grimeye - Orgrimmar)
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This is not a comprehensive list, it is a work in progress that I plan on updating as I continue my progress to 60.
Feel free to chime in with any quests you know of that could present a problem and I'll add them to the main post.