That an infant sleeping like a log in your arms will immediately wake up and scream when placed in his crib? No matter how gently you do it, no matter if you put a heated rice sack in there for warmth, no matter if you stand over it and go "SHHHH" until you run out of breath and pass out - none of this will work.
Some of the sounds, I swear you'd think we were torturing him...
My worst moment to date: I was clipping Owen's nails yesterday while Lisa was feeding him. Those things can get like razor blades, so we try to tag-team him to get it done quicker. I misjudged one thumb nail and dug into the tip of his thumb by mistake. A tiny pinpoint of blood and a little line on the flesh were all that you could see - but he screamed his head off! My heart sunk and I felt like the worst father on the face of the planet. I'd CUT my little baby boy! Talk about feeling crappy.