I was culture-shocked when one of Gambian friends told me that it is rude in her country. I thought it is something universal. I'm Indonesian and they're studying in Indonesia)
Few months ago, I replied "hm ëm" (yes / I understand) and the same lady didn't understand me or "what?" face, I said "hm ëmmm!" and still didn't understand and finally, she annoyed and I explained what I mean and she explained it
and before that, one guy (my friend) didn't understand as well and since whatever time, he imitated my "hm ëm" occasionally for joke like it's unusual for him, therefore funny