Yesterday, I was troubleshooting a corrupted SIM Tool instance export of Habitat. The exported ZIP file could not be imported back and 7-Zip identified CRC errors in 2 files.
The Web Forms for Marketers (WFFM) installation guide do not mention those files. I found out they are used to switch the storage location of form data from the xDB to an SQL database (see Use a custom SQL provider to store form data).
Since I did not need those files, I simply deleted them from my web root. The next SIM Tool export was successful and valid.