I just discovered a small error in the Maine county map boundaries for the elections 1980 - 2000. It turns out that the town of Otisfield seceded from Cumberland County in 1978 and joined Oxford County. I have now refelected this border change in the appropriate maps.
It appears to be a very rare occurance for towns to switch counties. I don’t know of another example for New England towns in the 20th Century. In 1890, New Shoreham, RI (Block Island) changed from Newport to Washington County. Anyone know of other examples?