The best way to learn something is to teach it. This paraphrased quote, attributed to Seneca, also applies to presenting, as you've probably experienced yourself.
When trying to explain something to someone who's not familiar with the topic, you often stumble upon gaps in your own knowledge. Things you took for granted are suddenly questioned; "Why, exactly, do we do this thing in this way anyway?"
The motivation for presenting is often to share what you know - and the joy of seeing others "get it". In order to be able to do this properly, you have to see things from your audience's perspective and to anticipate the questions they may have. Which is the point at which you'll notice that you're missing the answers to some of those questions. If you're passionate about your topic, this should motivate you even more. Fill the gaps in your own knowledge so you can prevent them from even opening up in your audience in the first place.
The risk here is that you may overwhelm your audience with all that knowledge of yours. You have to carefully select the content and the depth that you cover. If you're talking to an audience that's new to your topic, refrain from trying to tell them all about the exciting (to you!) details. Keep in mind that they're only hearing about all this for the first time and let them explore this new space on their own. Feel free to tell them a little about an aspect that you find most interesting about the topic, though. Share your passion; it'll help ignite theirs.
The other risk is that sometimes, you may be taking the "learn something yourself" aspect a bit too far. Don't do a presentation because you want to learn about a new topic yourself. When you commit to do a presentation, you should have some solid knowledge of the topic already. If you're doing it properly, enough gaps in your knowledge will show up anyway.
When preparing a presentation, you have to know where you're going with your talk. It's okay to know that something works in a certain way but not why yet - that's one of those gaps that you can fill along the way. But if you're not sure of the outcome before you start, then this is not a topic you should be talking about (not yet, anyway).
It's perfectly okay (and part of the fun!) to prepare a presentation, even if you find a few gaps in your knowledge along the way. But don't use a presentation as an excuse to learn something new; you need at least some knowledge about your topic before you should be telling others about it.
(Image credits: Slide by yours truly, using Writing on blackboard, iStockphoto file #15878546)
Please email me for details.