I find that there are certain trigger words that seem to elicit a certain type of response on Mumsnet.
If someone asks about sleep training, many posters assume that the OP is referring to cry it out, with endless posts about sleep training is just 'teaching a baby that nobody will come when they cry'. This is in spite of the many different ways of sleep training, many of which involve the parent staying with the child or very close by.
I've also seen a few times that posters who take an interest in current trends are perceived as simpletons who take all of their existing clothes and burn them on a big pyre, then replace the lot with a massive haul from shien, every year. No concept of adding in new pieces here and there to adapt to your current wardrobe.
Finally I've also noticed the misconception that it takes half an hour to do your make up in the morning, and many posters would rather have that extra 30 minutes in bed. I've seen posters explain time and again that it takes them 5 minutes to put make up on, but the myth that the poster gets up at 5 am to do her make up because it's so time consuming persists.
Just a little musing I thought of recently. Anyone else notice common conclusions you see people jump to on mumsnet? I'm sure gentle parenting is another!