I had a customer recently who had to fully resynch their data from Salesforce because of an unfortunate unrecoverable data mishap, and I thought it might be helpful to explain the process and mention some "gotchas" when considering this.
Marketo maintains a data value change log in the activity history of every person, so if the problem is due to a list import, it might be possible to simply re-import a new list, re-overwriting that data and correct it.
Another more advanced remediation possibility would be to identify the people affected, and pull the data value changes activities from Marketo. Adobe ACS consulting can help with this! We can pull those data value changes and give them to you in a CSV file so you can analyze those changes, and create the necessary "remediation import file"
In certain cases.. where the "new" and "old" values are known, ACS can also write a custom solution to revert those values, pulling them from the "old" value in the data value changed. An example of this might be if you imported a list and updated a locale from en_US to '' erroneously, and we can also target the source, such as "sfdc"
If, however, you must fully resync Salesforce data, the process will look something like this
(optional) export all person data, which ACS consulting can help with
Pause the salesforce sync
Delete the records from Marketo
Contact Adobe support to reset the sync "watermark" date which will cause a full resync
Re-enable the Salesforce sync
It's important to remember, though that you should refrain from making changes to the objects in salesforce during the process, and its always important to remember that If you want to remove a field from SalesForce, you should
Hide the field from the profile
Remove the field from salesforce
Remove any usage of that field in Marketo in smart lists or campaigns
Go to admin --> field management, select the field and click “hide field”