Chaos. Anything that doesn't have a set pattern for me to memorize, or at least repeat predictably.
So I run into problems when I draw things like ripped clothing, or uncontained liquid (especially blood splatters), or really long, loose hair that kinda just keeps going without any specific overall shape. I can do maybe five or six strands before I start feeling lost and not knowing what to do to fill up the rest of the space. :[
You probably wouldn't notice in most of my drawings, because I work really hard at making them look good regardless-- I will redraw a set of blood splatters 20 times if that's what it takes to get it right. But that's almost always what it takes, and it's exhausting...
I really envy those artists who are good at things like that, and can cover a page with just an anime girl's hair...meanwhile, just doing this sketch almost broke me: