Youtube has its trolls... but as for comments that are genuinely stupid, I would say it's comments that clearly show the poster didn't watch the entire video, read the description, etc. Those always make me facepalm, as the information was right there in front of them but they were too stupid to see it.
This happened a lot to me when I posted a series of videos showcasing the different animations of Pokemon attacks across the generations. I arranged the attacks in videos according to either type or effect, and at the end of each video, I included some comments about a certain attack being in a different video - i.e. in the Grass attacks video: "Solarbeam is in the Multi-Turn Moves video". I even included an index listing which video each attack could be found in. Yet despite this, I got a LOT of comments saying "You forgot (name of attack)!" or "Where is (name of attack)?" Sometimes when I specifically said in the video that that attack was in some other video. Sometimes I was generous and replied to them saying which video the attack was in, and also reminding them to check the index next time, but other times I got so tired of it that I didn't bother.
This happened a lot to me when I posted a series of videos showcasing the different animations of Pokemon attacks across the generations. I arranged the attacks in videos according to either type or effect, and at the end of each video, I included some comments about a certain attack being in a different video - i.e. in the Grass attacks video: "Solarbeam is in the Multi-Turn Moves video". I even included an index listing which video each attack could be found in. Yet despite this, I got a LOT of comments saying "You forgot (name of attack)!" or "Where is (name of attack)?" Sometimes when I specifically said in the video that that attack was in some other video. Sometimes I was generous and replied to them saying which video the attack was in, and also reminding them to check the index next time, but other times I got so tired of it that I didn't bother.