Okay, these are a rough indicator on what the side effects are gonna be. There wasn't really a list of side effects so I'm just going by what people have said when they did the spell.
Visions/ dreams of having wings/flying
Discolourations on your back at the end of your shoulder blades
Pressure on back (where wings are supposed to be)
Grey marks where wings are supposed to grow
Feathers growing out of your back
Feel like there is something growing in your back (this would most likely be the extra bones growing in your back because of your wings)
Thats all I got. I will update it when I do my spell