You may not have ever accidentally put the wrong fuel into your tank but it's good to know what to do in the event that you do. Mixing fuels doesn't have to be a catastrophe but you should try to avoid it from happening. Here's what you should do if this happens to you:
If you realize you've accidentally filled your diesel car with gasoline the first thing you should do is stop driving immediately. You will need to get it towed to a professional who can flush the system. Because diesel is also a lubricant significant damage can be done to the fuel-injector pump. Gas in a diesel car also causes misfires and knocking which will require parts of the engine be repaired.
If you put diesel fuel in a gas car the results are not as severe. You should still stop driving as soon as you notice the mistake, this will limit the damage done. The tank, fuel lines, fuel rails, and injectors will need to be drained. You should get a mechanic to do this work.