Run this code:
You should see a landscape, and after a second a large hole is digged below yourself.
The bug is that the hole is much deeper than it should: this is the code of my Control:
[java]heights.add(hLocal-0.0001f);[/java]
The hole should be barely noticeable, but instead is very deep.
You may ask, why am I using a Control for modifying the heightmap? Because I want to alter it on realtime with a smooth effect.
You are adding the current height minus a small amount. Adding instead of setting.
Thanks for your help, but I want to humbly point out that you are wrong.
If I replace my code with:
[java]heights.add(-0.0001f);[/java]
Then I get an even deeper and completely flat hole. In this case, I am really setting instead of adding.
always worked fine for me…
though I see you’re not using tpf in the statement so if your framerate is very high it will dig a hole very quickly
ie try
[java]
heights.add(hLocal-(0.0001f*tpf));
[/java]
or something to that effect.
Thanks for your help, but I want to humbly point out that you are wrong.
The Control has this line:
[java] spatial.removeControl(this);[/java]
Therefore it’s executed exactly once (and besides, I’ve already tried to fiddle with tpf with no success) 
Solved! 
I needed to do terrain.adjustHeight! Thanks for your help!