Obviously all you out there with little babies have already gone through this, since Karl is so "behind" in milestones ~wink~, but I'm just now dealing with this again and I completely forgot about it. I went through it with both girls and now it's Karl's turn.
It's when they learn to stand up, but have NO IDEA how to sit back down.
My kids have all been good crib sleepers. There's been exceptions: I had a hard time transitioning Ella from swing to crib for daytime naps; Allison woke up every night 3-11 times until the night of her 2nd birthday; Karl cries when I put him down when I could just put the girls in their crib when it was time for bed, even if they weren't ready to go down, and they'd just go to sleep. But all in all, I'd say they were (and are) all awesome.
When they learned to pull themselves to the standing position, they would do it a lot in their cribs. They'd be tired and needing sleep, but standing was such a great new thing that they would immediately stand up. Then they'd realize they had no clue how to get back down. The bending of the legs to sit down was (and is) always a mystery. So there I am, hearing Karl cry cause he's standing, and I have to go back in the room to lay him down, only to have him stand up again seconds later.
I don't know any other solution than to continue to go back in the room and lay him down until he stays down. Is there anything else to do?