Well, for me specifically, it was just kind of embarrassing to be the ignorant one.

Its like, I'm going over to my Muslim neighbor to see their birthday party. Later on... well... actually they were Sikh and I was ignorant and didn't know the difference until I asked them.

Fortunately, my neighbors must have been very familiar with the confusion, because they handled the discussion "what is a Sikh" pretty well.

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During 9/11 itself, I was in Catholic school. The teachers made it very clear that we needed to learn more about other religions at the time. So we visited local Churches, Synagogues and Mosques as part of various field trips. So I was familiar with Muslims, Jews, Protestants.

But we never visited a Sikh temple. I simply didn't know that religion even existed until I met my neighbors a few years later.

I think it's more like, if you assume that someone is Italian and then find out that they're Mexican, it's awkward that you made an incorrect assumption about their ethnicity despite the assumption being neutral in terms of judgment.

Yes, it seems that the weirdness of the mentioned interactions does not mainly stems from the confusion of the faith. It's based on people expressing their judgement of you as a person in relation to their perceived faith.