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http://mkv25.net/dfma/movie-458-treadingwater
I had a farmer fall in my moat after winter was over.
Might want to speed up the video, the rescue attempt took longer than I thought it would, partly due to my own mistakes. ;)
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...and that's why you should make ramps along the outside of your moat :)
I had a similar sort of problem with dwarves randomly falling into a pond that was in my meeting area in one game, so I built ramps at the edges for them to get out again...but I eventually got so fed up with the "Dangerous Terrain" spam that I built a bridge over the pond.