Here's one that stumped me for a while, though in retrospect it should have been obvious. I was getting the error message

NoMethodError: undefined method `constantize' for 0:Fixnum

when accessing a model through a polymorphic association. Turns out the table on the belongs_to side of the association had an integer type column instead of a string.

Easily fixed, but it seems like Rails ought to raise an error in this situation -- instead it happily adds the row with 0 in the type column.