It is easy to hit reply all when responding to an email without paying attention to the subject line, but why should we pay attention to it?
Email subject lines summarize the message, so the receiver can easily understand the topic and the importance of the email. But these fall short when they are non-descriptive or when the email thread has gone from a topic to a new topic without notice.
Business writers often suggest not changing the email subject line to increase searchability on your mailbox, and they are right. But it would be a difficult search for Topic B when topic B lies somewhere under Topic A email conversation.
Aside from searchability, you owe it to your audience to clearly communicate by making sure you change the subject line when you consider you are starting a new topic — regardless of the proximity of the topic.
And for short messages or quick follow-ups, avoid email entirely and instead use collaboration tools. Your recipients will appreciate it!