When you have a CSV file that has a column with a multiline value (eg it contains carriage returns), FileMaker fails to import the file correctly if that column is not mapped in the import script step.
Steps to reproduce
1. Create CSV file with following data:
ID, FirstName, LastName, Comment
1001,Mark,Peeters,"This is a nice place,
I would love to stay more"
2. Import this csv file into FileMaker without mapping the "Comment" column
5 records would have been imported
only the first 2 records are correctly imported, then the 4th is messed up, and maybe other records are correctly imported afterwards.