The messages similar to the following might be found in the xConnect logs:
[TIMESTAMP value] [Error] Sitecore.XConnect.Operations.SetFacetOperation`1[Sitecore.XConnect.Facet]: Sitecore.XConnect.Operations.FacetOperationException: Operation #18, Conflict, Contact {ID}, KeyBehaviorCache
[TIMESTAMP value] [Error] Sitecore.XConnect.Operations.SetFacetOperation`1[Sitecore.XConnect.Facet]: Sitecore.XConnect.Operations.FacetOperationException: Operation #19, Conflict, Contact {ID}, InteractionsCache
[TIMESTAMP value] [Error] Sitecore.XConnect.Operations.SetFacetOperation`1[Sitecore.XConnect.Facet]: Sitecore.XConnect.Operations.FacetOperationException: Operation #20, Conflict, Contact {ID}, ExmKeyBehaviorCache
[TIMESTAMP value] [Error] Sitecore.XConnect.Operations.SetFacetOperation`1[Sitecore.XConnect.Facet]: Sitecore.XConnect.Operations.FacetOperationException: Operation #21, Conflict, Contact {ID}, EngagementMeasures
The exceptions are thrown when calculated facets try to be updated. The conflict occurs if a contact has several simultaneous interactions saved at the same time. In this case, a facet is updated for the first interaction, while updating for the next interactions fails with an exception.
When xConnect attempts to save the value of a calculated facet, the operation is retried automatically. If saving fails after a number of retries, the Sitecore.XConnect.Operations.PatchFacetOperation exception is thrown.
The Sitecore.XConnect.Operations.FacetOperationException exceptions can be safely ignored unless the Sitecore.XConnect.Operations.PatchFacetOperation exceptions are thrown.