It will depend on whether it was an automatic or manual transmission.
Then it would depend on what broke.
If a clutch breaks you may see a lot of smoke and not be able to drive very fast.
If a pump breaks then you may coast to a stop.
If a linkage breaks then you may not be able to change gears OR you may have to select a different gear in order to drive at all.
If a gear breaks then you may hear some very expensive sounds and see some pieces flying around.
if a tie rod breaks you will lose control of the tire it broke on , causing you to crash..
What can cause the RPM on a car to jump while driving is a problem with the transmission. It could be a problem with the gears in the transmission itself.
Dirty oil and filter
May not read the speed of the truck while driving. Transmission may not shift correctly.
The driver will lose steering control. The wheels may turn in different directions, and all the driver can do is hold on until the car stops.
Your transmission might automatically shift out of overdrive while driving on the interstate because of low transmission fluid. This issue might also occur because of a bad transmission sensor or overdrive sensor.
The car will jerk, feel like there is something heavy slowing it down and then it will not move. It happened to me today and you will know it when it happens. The car will not be able to move until that axle is fixed (which is a cheap fix)
If you are ill and taking cough medicine while driving what happens
That depends on the engine and what belt (timing or accessory).
in some cases but not usually
Serious crashes.
On the vast majority of modern cars today it is impossible to actually shift to reverse while driving at high speed. Modern transmissions have an override safety device that prevents the transmission from going into reverse at any speed forward that would harm the transmission. You could put the selector in reverse, but the transmission would not shift to reverse. On older transmissions without this feature, you would probably destroy the transmission and possible have an accident.