What will happen if you accidentally put mtf into an automatic transmission? How will it act? How long before it goes kaput?

 

Definately not a good thing. How fast the tranny goes out depends on how much MTF was put into it.
If this happened to you, I suggest you do a full trans flush before driving the car any further.
Cheaper do do a flush than replace the transmission.

This is the recommended HONDA procedure. You will need 12 quarts of HONDA ATF (you can use another ATF, but it can actually make the transmission shift firmer than before, and some people don't like that).
NEVER ATTEMPT TO CRAWL UNDER A CAR WITH JUST A JACK HOLDING UP THE VEHICLE!

1) Put the front of the car up on jackstands with the wheels off the ground. Put a large container under drain plug.
2) Remove the transmission drain plug (3/8" drive ratchet works fine). Let transmission drain and wipe sludge off drain plug.
3) Reinstall drain plug and snug (not tighten). Fill transmission with three quarts of ATF.
4) Start engine and shift into drive. Gently accelerate and let transmission upshift through the gears (don't go over 50 MPH).
5) Step on brakes and bring speedo to 0.
6) Shift into reverse and accelerate to 20 MPH.
7) Bring speedo back to 0 with brakes. Shift to park and shut off engine.
8) Repeat steps 2 through 7 TWO MORE TIMES.
9) Drain the transmission once more. Reinstall drain plug and tighten firmly (don't forget to reinstall crush washer).
10) Remove jackstands and put vehicle on level ground.
11) Fill transmission with 2 1/2 quarts of ATF. Start engine. Shift to drive, then reverse, then put back in park and shut off engine.
12) Check transmission fluid level. Top off if needed to bring fluid level to the top of the crosshatch marks on the dipstick.

Disclaimer: (This is for my protection) This information is for informational use only, and the writer assumes no responsibility whatsoever for any damages or injuries to any vehicle and/or person who chooses to use this information.

 

If my tranny has mtf in it, how will the transmission act? Will it slip? Will it shift harshly? Will it shift at all? ect...

 

It could do all that, Really its hard to say. I would Imagine it Will Destroy youre clutches or you Torque converter.