I'm very used to terraform, yet I sometimes have a damn ugly annoying problem.
When I select the "level" tool of terraforming and press alt and drag mouse over map to terraform a leveled terrain... On some maps (or just sometimes?) I have no problem, the terraform will be perfectly plane as it should. However on other maps (or other times?) the tool doesn't work correctly. Instead of a plane terrain, it terraforms like a rectangular plane area but not filled. So it's extremely hard to terraform terrain into a somehow big plane. I'd have to terraform tons of times the rectangular shape until filled all the middle of the first rectangular.
If you don't understand what I'm saying I could show you with a screenshot, next time it happens.