I see this happen on a lot of mailing lists, but I just spotted a 13-message thread on this list in which several totally different subjects were discussed.
Some mail readers "can thread" mail such that all messages that are replies to each other appear as a single entry in the list of messages. I do this to speed mail handling, since I get hundreds of emails a day. I will see the subject line of the first message and the number of messages that follow from it. If I delete, I will delete the entire thread, with the assumption that it is all about the shown subject. If you use the Reply function of your mailer to start a new topic, chances are I and several other people will never even know about your message because it will show up as just another part of the thread you actually hit Reply on. Example: If you want to ask how to right-click with VoiceOver, but for convenience, you do it by replying to a message entitled "How well does Fusion work?" I'll just notice a multi-message thread called "How well does Fusion work?" If I don't want to read about Fusion, I'll delete that thread never knowing you tossed a completely different subject into it. Note that changing the subject line is not enough. Mail programs put an "In-Reply-To" header into a message when you use Reply, and that's how messages are threaded in most mailers that do threading. So in summary, don't use Reply if you're starting a new topic, or some of us won't even see your message. I know Reply is convenient, but its use when you're not really replying might cause inconvenient side effects.
