[Imp]

Good luck with that I have noticed the editor does on occasion leave a partial grey hex but has not bothered me. May be caused by using fill tool extensivly at map edge.
One thing I have noticed is rail lines seem to cause problems at times possibly linked to pavement as follows.

If doing city so place rail lines & or pavement first to get general layout area within 2-3 hexes of rail becomes problematic.
Cannot place buildings sometimes trees to in certain hexes.
In some instances clearing the pavement from the hex then allows placement but its hit & miss.
I am not talking multi hex buildings which rail could possibly prevent but single hex buildings.
On a very odd occasion it gets confused & chucks up an invisible slope in an area
Liveable with if annoying at times as have to restructure but I am guessing the code for terrain is interlinked & finding whats going on could be fun & might break something in the generator which is a great tool. All this flexibility you provide the end user carries the price of something else to house a glitch as even a little change seems capable of having unexpected results