All learners of Chinese, or just the ones whose native language is English?

I figure the most natural mistake to make when counting pants is to use 件, since that's the measure word that applies to "clothes", but you'd have to already know at least some Chinese in order to make that mistake.

Ah, the ones coming from English of course. I don't have much experience in how Koreans or Russians learn Chinese (although I had plenty of non-English classmates when I took language lessons at PKU).

I'm thinking more Kouyu than writing. If I didn't think hard about it, I might say yi shuang kuzi or even, for some reason, yi jian kuzi.