When a command alarm can't be executed (e.g. if the command to execute can't be found), in addition to the error message saying that execution failed, an additional, unnecessary error message "Failed to update alarm" is displayed. This only happens with a non-recurring alarm.
The reason was that the alarm was deleted before the command execution result was known. The alarm's execution status was then updated, but the alarm no longer existed, so the error was produced. KAlarm now checks if the alarm has already been deleted before attempting to update its status. KAlarm version 2.11.13, commit 6cd6bbae49a4eff15da8d46e11f253b00245693c (Applications/16.12 branch).