This doesn't happen all the time, but it happens once in awhile. It's enough to cause confusion and annoyance.

When trying to sleep in a bed, the chat window and leave bed button appear while the 'getting into bed' animation is stuck in a loop in the background. It seems as if you are in bed, but will prevent the night from skipping to day. You have to get out of bed first, then enter the bed to be able to actually sleep.

I have only seen this on SMP with another player logged in, however it has happened to a friend on the same server without anybody else logged in. I do not know if the player model is displayed as 'in bed' or just standing, but the client-side display does update as if you are sleeping, just with the loop in the background. If it happens again, I will try to check this out.

Possible cause: Still moving while trying to enter bed while there is some lag on the server. My friend suggested this, as this issue has affected her more than it has me.

EDIT:
I can reproduce this following these steps:
1: Stand next to bed preparing to right click
2: Toggle Wi-Fi switch on/off
3: After the connection to the internet is lost, right click the bed and hold down the right mouse button
4: When the connection is reestablished, the sleeping interface should come up and the animation will start looping