If you suddenly start getting an “incompatible marshal file format” error when trying to insert into your database, check to see if you’re using acts_as_indexed.
If you are, it’s quite likely that your index has been corrupted. The fix for me was to go nuclear on the tmp/index directory. Deleting the entire thing worked like a charm. I supposed I could have just removed the subdirectory pertinent to the model in question, but what fun that?