If you are running SQL Server 2016 (especially before CU3) you have received this error:
DATE/TIME: 8/21/2017 11:24:53 AM
DESCRIPTION: Always On Availability Groups transport has detected a missing log block for availability database "Database". LSN of last applied log block is (99939:95847:0). Log scan will be restarted to fix the issue. This is an informational message only. No user action is required.
JOB RUN: (None)
I’ve talked with the product team about it, and its just something that happens, and is more of an informational message. Based on some discussions I’ve have on #sqlhelp on Twitter, it may be related to not enough network bandwidth between nodes. But if you see these sporadically, relax, it’s nothing major.