This may be an issue from the past, i checked past 5 pages and didn't see it, it may also be an issue with minecraft in general that can't be easily fixed. That being said...

It's possible, inside of a protection stone area, to jump and place a block. The block will disappear because you can't build there but the game still thinks you are standing on a block so now you are hovering 1 block off the ground. This can be done repeatedly until you are high enough into the air that you can just jump over any wall in front of you that doesn't have a roof of some sort.

I currently run essentials full, preciousstones, spigot, worldedit, and mcmmo. I have also confirmed that this can be done with just bukkit/spigot and preciousstones. Currently an issue in 1.6.4. Not sure if 1.7.2 will change it or not.

Block lagging is a byproduct of the lag between server and client. Nothing to do with PS.