I was trying to add a fire effect to the side of this car. I wanted just the fire to be showing and everything else transparent. I made the node setup. But for some reason, the transparent areas were not 100% transparent. The transparent areas looked a little brighter than the actual area. I thought it was my node setup that was the problem and lost my mind over this. Then I said frick it and plugged in just a transparent shader. To my surprise, with no other node setup connected, just the transparent shader was not in fact fully transparent. So it's definitely nothing to do with my setup.
I opened up another blender file and made a cube and a plane in front of the cube to test my theory. I was right!
This time, the transparent shader made the area a little darker. I tried some things people recommended online but nothing worked.