As far as I'm aware there's only one legitimately missable event. There's a beach scene, and not long after, there's a one on one scene where you get tickets to some indoor resort. You can pick any of the three starting girls to go, and once you lock in your choice, you can pick the others from the gallery and see their scenes that way. If you see the scene for the first time while NOT being a secret lover with one of the girls though, you're permanently locked out of it. I hadn't reached that point with Rai yet and now I can't see her version of the scene unless I start a new game, even though she's now also a secret lover. I'm missing a few other events that I SHOULD have met the prerequisites for seeing, but I'm assuming it's a glitch or something.
Skill CGs aren't missable, every time you use a skill you randomly get one of the CGs from any unlocked level.
Ah thanks. Good to hear that the skill CGs are randomised so that theres some variation and no missables in that aspect. What's the conditions for Rai secret lover event? Just got the beach event but everytime I try to talk to her after work, its just the generic hang out.