Open mouths and anything inside them, eg. teeth. 
I guess I kinda never really bothered learning how to draw these properly, and sadly it's showing, even just a generic open mouth is hard for me to draw, because I can't get the shape right, it just looks weird, and more often than not ruins an otherwise decent face drawing.
Other than that, hm... I find the entire jaw/neck/head underside area very tricky to draw, and have trouble visualizing it. It's not only an issue in low angle shots, either. I just struggle to understand how it works in 3D, even with reference.
To be honest, there isn't a part of the face/head I find easy to draw, but yeah, these are the worst 