starting from flawed premises (aka "The world is a just place" or "a just god exists")
Inexperienced edgelords
Arguing in bad faith (or just "I've got mine, so fuck you")
I say this as someone who flirted with libertarianism in my early 20's due mostly to a conservative religious upbringing. I'm not super smart, but I flatter myself to say that I'm above average intelligence and education, and there was still a brief time in my life when libertarianism served like a valid ideology. It took actually reading some objectivist philosophy and simultaneously reexamining and then rejecting my religious background before the flaws became evident.