An ETag is a mechanism for optimistic concurrency control. The bot data bag storage uses ETags to prevent conflicting updates to the data. An ETag error with HTTP status code 412 "Precondition Failed" indicates that there were multiple messages received in parallel before the bot could finish its first operation. The dialog stack and state are stored in bot data bags. For example, you might see the "Precondition Failed" ETag error if your bot is still processing a previous message when it receives a new message for that conversation.
