1999 Saturn SL2 accelerates when hitting the braks

I don't understand this one. When I approach a stoplight and hit the brakes,
the motor starts revving up, as if I am hitting the gas pedal.
Another thing; my "check engine" light came on as soon as I left one of
these oil change places. I thought maybe a wire had been accidently knocked
of from a downstream sensor until the mechanic explained that there is no
downstream sensor.
OBD problem..........................O2 Sensor, slow response. I went down
into the oil change pit so I could get a better look at things. The mechanic
explained that this vehicle has only one O2 sensor, and that nobody had
knocked any wires off. It could have been a fluke. Is it true, that there is
only one O2 sensor on a 1999 SL2?
I am not sure what to make out of this one. Has anybody seen this problem?
Thanks in advance!

Pete

 

does it happen all the time? If it happens more with one pair of shoes
they may be the problem... (I notice that I can heel-toe much better in
tennis shoes than boots or sandals).

There are 2 - one in the manifold, one after the catalytic converter.

 

Most likely there's an air leak in the hose to the brake power
booster.
Check this first.

 

Thanks folks! I'll check these things out.

Pete