[ohsnap]

when someone uses a personal experience to bolster an argument, it is appropriate for someone to question the validity of how likely it is. It's not an ad hominem or a personal attack.

[notjoedimaggio]

Perhaps. But "pretty sure you're making this up" is not an appropriate tone or argument to question validity. Especially when the basis for your argument is your singular person experience. Your never having heard the argument in no way counters the original assertion that their instructor had students make that statement, unless you attended every single course and class the original commenter refers to.

Next time, perhaps phrase your argument around your experience. "While I don't know where you're from, I never experienced this. Where I grew up in XYZ, people were quickly corrected regarding their misconceptions."

[hueving]

Your reply was just essentially the same thing in reverse, "I've never seen this therefore it hasn't happened."

[ohsnap]

Actually his/her comment was anecdotal, in that no one can verify. My comment is a noting a shared cultural understanding that everyone confirm or reject. (specifically: the likelihood of people saying your classmates are genetically inferior is taboo) It's not anecdotal.