I think it's when she starts sleeping in the big girl bed. :'(
Last night was her first night sleeping in the big girl bed, although she took a couple of naps in it over the weekend. We tried her on it during a nap the weekend before, but she kept coming out, so I gave up and put her in the crib to nap. This past weekend, she kept coming out again, so we got a bungee cord and tied it to the adjacent door so that she couldn't get out (it's like a crib, but bigger!). That seemed to do the trick and on Sunday, she didn't come out at all. Someone else told us in the past to reverse the lock on the door so that it's outside rather than inside and that's a pretty good idea, but more effort than we were willing to put in.
I feel like it's bittersweet how quickly she's growing up.
By the way, that's my childhood bed. Which became my brother's. Which became ours when my mom moved. I feel thrifty thinking about how we didn't have to buy any furniture for that room. Ha.