I am so used to using Google maps with the satellite view (with the streets and labels) that I just noticed on the 'standard' map of Portland that Monument Square is labeled as a shopping center. Also, it appears to have a pond! (David's Restaurant is misplaced, too)
Another odd spot is the "Spring Street Historic District" in the West End. The green area signifying this district is actually the playground and fields for the Reiche School. Perhaps ironically, as Strange Maine documented a while back, the area now occupied by the Reiche School was once a bustling neighborhood that might have become a historical district at some point.
There are countless other oddities (can anyone else find the mysterious pond on Munjoy Hill?) that caught my eye. I love Google and their maps but wish I knew how to suggest improvements. In addition to the correction of various labels around town, I would like to see building footprints added, as seen in many larger cities.