"Good thinking ... It's often infested with Nargles."
-- Luna Lovegood, as Harry jumped out from under the mistletoe (OP21)
Known to infest mistletoe quite often (OP21).
When Luna and Harry were alone in the Room of Requirement waiting for the meeting of Dumbledore's Army, mistletoe was hanging over Harry's head. When he noticed that Luna was staring at it "dreamily," he jumped away from it, causing her to mention Nargles. Later, he and Cho Chang were alone under the mistletoe, and this time Harry brought up Nargles out of nervousness, but Cho laughed and they kissed (OP21). Hermione later asked if Cho "cornered him."
Did Dobby really decorate the Room of Requirement with the mistletoe, or did it appear because Luna wanted to kiss Harry underneath it? After all, it was the room where someone can get what they want, and she had a "dreamy look" as she stared at it. Luna may have quickly mentioned the Nargles (real or imaginary) because she was embarrassed or disappointed that Harry avoided kissing her. Later when he and Cho Chang were alone under the mistletoe, Harry started babbling about Nargles out of nervousness because he had never kissed a girl before. Unfortunately, he also called Luna "Loony" which made Cho laugh, and that led to his first kiss. In many ways, this was written as an awkward teenage situation for everyone involved.