This is really stupid, but it seems like enough of a stupid niche problem that it might be worth mentioning.

There's an integer column in a database that maps to an enum. Whenever I tried to mark that enum as a column, I got an InvalidCastException (from deep within the Linq reflection-magic call stack). No useful information.

So I wrapped an int property around that enum and specified that as the column.  Same result.

After much frustrated googling, I double-checked the schema and realized that the column type was actually a bigint.  Even though the value was only 8, it couldn't even handle that much of a cast.