If you put fabric softener in the laundry detergent portion of the washing machine then nothing bad will happen.
What will happen though is you just washed your clothes with fabric softener instead of laundry detergent and in that case you'd need to rewash your clothes again and put laundry detergent in the laundry detergent compartment of the washing machine and Fabric Softener in the Fabric Softener compartment.
Fabric softener only helps to soften your clothes during the final rinse cycle because if it's added during the wash cycle the fabric softener will be washed away.
So if you put fabric softener in the detergent compartment all you need to do is just rewash the clothes again with laundry detergent and your clothes and washing machine will be fine.