I've never owned a vehicle with an auto tranny; both of my
current cars have manual trannies.
Some years ago, a group of friends and I were driving out of state
to attend a convention. It was an 8-hour drive; we were riding in a
friend's Honda Accord with an auto tranny, and it was decided that
we would share in the driving chores. When it came my turn to
drive, everything was fine until we entered a small farming
community and approached a four-way stop intersection. As I was
coasting to a stop, my left foot instinctively extended to push in
the non-existent clutch pedal, and found the left edge of the
wider-than-normal brake pedal instead.
This had the immediate effect of locking up all four wheels--I
didn't think to release the "clutch" pedal, as it's normal practice
to keep it depressed to disengage the tranny when coming to a
complete stop.
After we skidded to a stop, it was rightfully decided by all aboard
that I should be relieved of driving duties for the remainder of
the outbound and return trip. 
Moral of the story: do not assume that a driver of a car with a
manual tranny can seamlessly transition to driving a car with an
auto tranny. 
