In no particular order:
1: I have to agree with EoSD metal fatigue bullets, took me some time before I learned to fly THRU the effing big bullets, and not around the medium ones on Remilia's opener, as well as that demon lord spellcard (whatever it's called)
2: EoSD knives. I swear the hitboxes for these things changes, it doesn't represent the circle hitbox you actually see in the PCB knife bullets IMO
3: Kanako's rice-whatever spellcard bullets, They're like metal fatigue bullets. Just thinking about going near one will kill you, seems more forgiving on hard than normal, haven't tried lunatic yet.
4: I like to call them "cucumber lasers" Because the first card where these really sucked for me was on Nitori's cucumber whatever its called spellcard. I don't know why, but I just can't seem to see these approaching me at ALL after I dodge an initial wave. It's like they camouflage each other or something. Another example are Yuugi's lasers
5: I forget her name, blonde short haired second last boss in UFO (shou something? I really forget), those damn curvey squiggly snakey lasers from hell. I mean seriously, can anybody read the trajectory of those until it's too late without luck? UGH!