End Crystals summoned in Flatlands still do terrain damage
This is a new 1.9 feature that most have just learned about:
If you place an 'end crystal' item on an obsidian/bedrock block, it will spawn an end crystal.
I did this in my flatlands plot, but didn't like where it was, so I punched it & didn't expect any damage since greifing is not allowed; however, this did damage my structure when it exploded. I, personally, believe that end crystals should only be decorative, and not be removed, but the explosive risk is something that should be taken care of (because of bad implications). I'm not sure if this is supposed to happen or not, so just a heads up for anyone who wants to play with end crystals.
I an not 100% sure if this is a problem. Finally you destroy your own stuff.
Also, the only way I would be able to block this is preventing anyone from using end crystals - anywhere in flatlands.
There is a theoretical risk that you could make an explosion on the border of your lot and take some of the neighbor lot with you. There might be a fix for that once the new version of WorldGuard comes out (no idea when that will be though).
Anyhow, griefing is not allowed as you say, but that does not mean it's impossible :)