I've done it... you've done it ... almost everyone I know has fallen through the ground at somepoint in time. Here is what you need to know to get out on top of this.
When this issue occurs your still stationary in the world. However you get out of sync with the server and you start to "fall". This is noticed by a large light blue background and in some cases you can even look around to see the objects flying above you.
What to do:
DO NOT "/stuck" ( you will just die and it will not correct the sync , and you might have to do it again after your sync'd backup)
Simply log off completly (close the client) and then log back on. One of two things will happen.
Situation A: You are resync'd and you can continue on.
Situation B: You are resync'd but the background is still light blue. You at this point will need to "/stuck" to get out. You will take a death but at least it's only one and you don't have to worry about two from doing it to early.