Hello,
This is really just a curiosity/what am I in for question since I've already taken my car to the shop.
I drive a 1999 Honda Civic LX(automatic) with 137,000 miles. Yesterday I was driving around and it downshifted on me from 3rd into second, causing the engine to rev, it lurched real bad on me then shifted back into 3rd and kept going. It did this to me a couple of more times on my way home. When I get in I checked the transmission fluid and it was pretty much dry, I ran out to the auto parts store(different car) and picked up some ATF and drove it around the block, it did fine for that stretch and then was fine also when I took it in to the shop this morning. At this point I'm just waiting on a call back from them to see what the diagnosis is. Is it possible this was just low fluid and won't be much to fix or am I probably looking at a bigger problem?
Thanks

 

It probably didn't downshift, it slipped / disengaged because of lack of pressure from the pump being out of fluid.

Here's what I'd do:
1. Get the car away from that "transmission specialist" ASAP. If they haven't done anything to it at all yet, that's best.
2. Find and repair where the fluid is leaking out. Your general third-party mechanic can do that if you don't work on cars yourself.
3. Drain and refill with Honda ATF. Do not use generic fluids or additives.