Another issue with slipping the old 40 degree flap 172s and even 150s, but
to a lesser extent, is tail stall. The pumping of the elevator that someone
else mentioned is incipient tail stall. I've had cases where this was a
suspected cause of accidents. You can also induce it in a Cessna 170B,
which is the 170 version with the modern wing and 40 degrees of flaps.
The recovery from a tail stall is counter-intuitive also - it takes back
pressure on the yoke, not forward pressure like you do to recover from a
wing stall.
Typically, as most of us know, the tailplane provides a downward force. But
in certain aft loading situations, it can provide an upward force, like the
main wing does.
When it is providing a down force, and if the tail stalls, the result is a
sudden pitch down that can be violent and catch a normal pilot totally off
guard, as he's never experienced a tail stall before.
That usually occurs with full flaps in high wing Cessnas, due to the airflow
that someone previously described when slipping; hence, one more reason that
Cessna restricted flap deflection to 30 degrees on the 152 and later 172.
Jerry E.