I used an automated customer service system that requires reading off one's case id. The case id I had included the sequence "3F."
The speech recognition software was only tripped up by that "F," which it asked to clarify if it were an "S" or an "F." I am assuming it picks the n-most likely phones and asks the user to choose when it meets some threshold of uncertainty.
When I spoke to an actual human (on the phone), she also asked if it were "S" or "F."
My conclusion is that having a word which begins with a interdental voiceless fricative causes a following labiodental fricative to be confused. There were other "F"'s in the id, but these were not confused (nor did they come before any interdental sounds).