1. Car Jumping
Every time you accelerate it will feel like the car loses power and jolts backwards and forwards as the petrol sticks to the pistons and then acts as a solvent causing damage to the pumps and corroding the lubrication which in turn means you have metal on metal, this can cause long term damage, but not if you get it drained quickly. There are many parts of a diesel engine that is not compatible with petrol, these parts can also be damaged too.
2. Smokey Exhaust
3. Engine Misfires
It's not all bad news....
If you have just had a fuel drain, then we recommend that you change your fuel filter afterwards just as an extra precaution, but again, you should be fine as long as the fuel has been drained thoroughly and the engine flushed through too.
The plus side, your car will be ok, don't read the horror stories on forums about needing a new engine, or your car being written off, it's a simple fix.