A generic error may occur, when there is a issue posting the record to Xero, and the error results in an unhandled error. Previously this was presented as:

Error Updating Xero: Xero Validation Error: An error occurred in Xero.
Please contact api@xero.com.
Also check the API Status page http://developer.xero.com/api-status/ for current service status.

As this error is related to the API, contacting Xero API Support would result is an extended support cycle, where we already know many of these generic errors and how to resolve them.

The following list may help you, with the most recent error appearing at the top of the table, and whether the issue has been resolved.
Error Code Issue with Record Problem Summary KB Article Resoloved / ETA
Item Account Code Missing from Sales / Purchase. Xero Inventory Item When posting a record to Xero, an Item is missing a Account Code in either Sales or Purchase which results in an unhandled exception. Xero Inventory Item Logged defect Xero API: 
Item Description is too Long Xero Inventory Item When posting a record to Xero, an Item has a sales or purchase description that is > 1000 characters. Xero Inventory Item Logged defect Xero API: