There are certain gross things that I understood about parenthood. I'd be changing diapers, lots of them, many containing poop, for years. I'd get peed on occasionally. I'd get barfed on occasionally.
Things I didn't expect would be voluntarily catching vomit in my hands because it's easier than cleaning it up off anything else. I also didn't expect that my breasts would leak like they do. I sometimes wake up having soaked a big puddle on the bed. Sure, there are breast pads that will soak up leaked milk, but in the middle of the night, they may get out of place and not be noticed. And guess what, breast milk is just like cows milk, it stinks when it spoils, and in fabric it spoils.
I probably even wrote about this with my first kid, but it's still gross and still unexpected, and I don't feel like I ever get to talk about it out loud.