Do you know what you're talking about?

Forgive me for being so blunt, but I have a very simple view of speeches. Imagine two circles on a page. One contains all of your knowledge. The other contains everything that your audience is interested in. If those circles overlap, then you should deliver a speech to that audience, on the topic(s) in the area that overlaps. If the circles don't overlap, you shouldn't even consider delivering a speech.

Of course, you wouldn't be as foolhardy as some speakers I've heard, who have no connection with their audience at all. I'm sure you only speak when you have a topic that is of interest to your listeners. However, problems can arise if you stray out of that overlapping zone, and start talking about things that you don't really understand, in order to please (or rather in an attempt to please) your audience.

From time to time, all speakers can suffer from the "edge effect", where they reach the borders of their expertise, but then stray over the line because the audience seems to like it. In fact, it's being disrespectful, and could lead to disaster, especially in any Q&A session. The best way to avoid the problem is to ask the event organiser to give you contact details for a few likely audience members, and have a chat with them a few weeks beforehand. Then base your speech solely on the overlap of your knowledge and their interests. It works every time.