This is starting to become a regular occurence whenever limsa is bustling with people it seems. Usually, walking through the aftcastle, or the market place on my way to the arcanist guild, i will fall right through the ground. I then find myself walking around on water. It was funny at first when i though it was a totally odd occurence...until it happened again...and again...and again. Basically, if limsa has alot of people in it, i can't walk through town without falling through.

To be more specific about the fall through locations...

Aftcastle - about mid point around the plaza.
Markets - usually right in between the two market halls where the company chest is at or in between the market and the arcanist guild.