When replying to a tweet, that tweet can be deleted while it's being replied to. Wen can catch this in some cases (namely, when the user is following the author of the tweet I assume, unless those rules for the streaming API are vastly different) and inform the user that sending the tweet is not going to succeed anyway, as per https://dev.twitter.com/overview/api/upcoming-changes-to-tweets:
In cases where the original Tweet has been deleted, the reply will fail. This is a change to existing behaviour, where it has been possible to reply to a deleted Tweet ID.
When replying to a tweet, that tweet can be deleted while it's being replied to. Wen can catch this in some cases (namely, when the user is following the author of the tweet I assume, unless those rules for the streaming API are vastly different) and inform the user that sending the tweet is not going to succeed anyway, as per https://dev.twitter.com/overview/api/upcoming-changes-to-tweets: