When the GA creates the transfer certificate, it gives it a one year validity range. It uses midnight UST of the current day as the valid-from date and one year from that date as the valid-to date. The leap day bug is that the GA calculated the valid-to date by simply taking the current date and adding one to its year. That meant that any GA that tried to create a transfer certificate on leap day set a valid-to date of February 29, 2013, an invalid date that caused the certificate creation to fail.
As mentioned, transfer certificate creation is the first step of the GA initialization and is required before it will connect to the HA. When a GA fails to create its certificates, it terminates. The HA has a 25-minute timeout for hearing from the GA. When a GA doesn’t connect within that timeout, the HA reinitializes the VM’s OS and restarts it.
If a clean VM (one in which no customer code has executed) times out its GA connection three times in a row, the HA decides that a hardware problem must be the cause since the GA would otherwise have reported an error. The HA then reports to the FC that the server is faulty and the FC moves it to a state called Human Investigate (HI).