I had a friend, in almost every other way a fantastic guy, who had a really frustrating habit of dismissing opinions that differed from his in political debates with "You're just brainwashed by the X" (removing all ideologies from the example to avoid derailing with a political debate). I found it incredibly dismissive and patronising, and as a result began to hate it when group conversations turned towards politics.
Then, one day the group were discussing sexism and feminism, and as a passionate feminist and also the only girl in the group, I was very enthusiastic about the conversation. I actually without realising responded to a point about men experiencing sexism as well with "Well yeah, that's patriarchal backlash, you've been brainwashed by the patriarchy into accepting it." (My mirroring of friend's comment was not intentional, I have a habit of picking up friends' phrases).
Friend later said to me that he hadn't liked it when I said that! I as nicely as possible stated that I didn't like it much when he did it to me, and he apologised. Turned out he'd picked it up from his dad, and he admitted he didn't like it when his dad used it either. So sometimes know-it-alls don't know they're being know-it-alls!