So you have just finished rendering and you can see fireflies in the corners of your room? This is most commonly caused by a combination of using an extremely bright external light source and your mesh has no thickness to it whatsoever. This can easily be fixed by either modeling your mesh with thickness to it or in this example, by adding a cloth surface to our room and increasing the thickness from 0cm to 1cm. Also, remember to make the subdivisions 0. There is no need for them in this case.
Examples
This is our default room with zero thickness on the walls. Note the faint white pixels in the corners - Click to enlarge
The same room but with the cloth surface applied. Note that the fireflies are no longer there.