Brilliance
He tells her the plan, and waits for her to admire his brilliance.
He should have known better.
First of all she frowns. Then she says, "You'll get into trouble again, Lysander."
Then she points out everything that is wrong with his brilliant plan to get revenge on Frankie Longbottom for her horrible prank involving niffler dung and his sock drawer, and exactly why it won't work, and how he could improve it. And she finishes with, "And the whole thing's stupid anyway. Frankie's your friend, and you'll only both end up in detention. It's the worst plan I ever heard."
Which annoys him, because he thinks his plan is great, actually, and anyway she's obviously completely missed the point, so he stalks off in disgust to go and muse on his brilliance without any wet blankets to ruin it.
An hour later, he has come to the conclusion that maybe his plan wasn't quite perfect, and that maybe her ideas will improve it slightly.
The following evening, when he and Frankie are both sitting in detention, he wonders whether Molly Weasley is ever wrong.