There really should be a way to notify someone if a record sync failed, especially considering Marketo gives up after the first try.
An example of where this is extremely prevalent is for SFDC integration customers who have validation rules setup. Validation rules related to Leads/Contacts are fairly common is SFDC, but they are often setup over time by an admin as they are needed, especially when new custom fields are created. A by-product of this process is that existing database records that were in SFDC don't have values in the required fields, so from the moment a validation rule is setup and going forward, when the record is being edited by a user it won't save unless that required field has a value. Since Marketo needs a SF user login, it is subject to these validation rules.
The example above may want to be documented in the SFDC installation/integration guides as well, since validation rules are not necessarily part of permissions and can apply to an admin user. We had no way of knowing, and luckily we caught this before we launched our first nurture campaign... otherwise thousands of records wouldn't have synced anything. *gulp*
I should add that a sync failed notification would be useful in more cases than the specific example above... API limits for example. I'm sure there are other examples that cause a sync failure.
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