וְהַר סִינַי עָשַׁן כֻּלּוֹ
As רש"י writes on this pasuk, there is a very distinct difference between עָשַׁן and עָשָׁן. The true meaning of the פסוק is that הר סיני was smoked i.e. covered in smoke. If read incorrectly with two קמצים it would imply that the mountain was all smoke. In short, the difference is whether the word is a verb or a noun.
If a בעל קריאה is not clear in differentiating his קמץ from his פתח (I've met a few of those in my day,) then this error might go unnoticed.