Importing contributions: "Expected one Contribution but found 25"
In the absence of clear error information, I went into trial and error mode. 

I disabled each mapping in turn. In the end, I found it was the attempt to import "contribution status" that triggered the failure. It is likely that the contents didn't match required values in the database. Though, in this case, the error message seemed to have nothing to do with that. My previous experiences with similar problems generated more clear errors that the content of the field was invalid.