Let us not forget that hydraulic jacks can fail at a moments
    notice!  Pack your jack-stands, a bunch of bricks, whatever to put
    under there as well.  DO NOT TRUST that floor jack.  I've seen one
    fail catastrophically , and heard several other horror stories..
    (the seals gave way,and they just collapsed!)
    Really an eye opener!

Having done almost all my work in the street,
I can also report that watching for idiots
is part of the job.  Was once under my '66 bug
when this car pulled into the space in front
of me and backed up -- knocking the bug off
the jack stands.

Luckily I saw it coming.  I came out from under
the car with a wrench in my hand (out of character,
but a good idea at the time) and, seeing me, the
perpetrator pulled away, shearing off the perp's
left front turn signal on the car in front.

I got weak knees and sat down.  

--dan

"Diplomacy:  The art of lying in State"