If you cover all of a puddle with sandbags and then start to hammer them down, some tiles may have their water removed, or they may leave behind glitched, perfectly square sections of flooded land. Screenshot below.
Edit: These tiles generally disappear once the "eye" of the puddle has had its sandbag hammered down. For some reason, these didn't. It may have had to do with the fact that the two puddles were right next to each other, nearly overlapping. Still, whether they disappear or not, this doesn't look right at all.
Cover puddle with sandbags, then hammer them down.