My C 150, with a Horton STOL kit is capable of sustained flight, and 15 degree banked turns at 22 MPH indicated. However, when I installed a swiveling pitot head and second ASI, I found that the actual airspeed was 41 MPH. So that would be a case where the position error on the pitot tube was extreme, and the difference between IAS and CAS was huge. But, it's fun to look at 22 MPH, and think to one's self: "wow, that's slow!".
Careful doing that for sustained periods though, as the CHT gets hot, and could be damaging.