It's OK to make the occasional mistake. You're only human. The occasional stumble is acceptable. Too many and you should do a re-take. One of the worst problems with narration, however, is sounding monotone. Before recording, practice varying your tone and adding inflection where appropriate. This makes the screencast infinitely more listenable. Also, project confidence through your voice and this will engender confidence in your students, as well.
Often, we do not realize how fast we talk. With screencasting, good pacing is essential. Speak clearly and practice talking slower than you think you should. However, long pauses should be avoided as the learner may think there is a technical problem on his or her end. Have you ever listened to a screencast where the narrator seemed to go and on . . . to the point of rambling? This is what happens when one lacks a concise and focused script or at least a bulleted outline. Stick to the script.
Keep your screencast as brief as you can and remember that chunking is an option if you have a lot to cover. You may be interested in reading the Hazellet article below which talks about the optimal time to maintain learner interest is about six minutes!
Lastly, put a smile in your voice. You will discover that it does make a difference in your delivery!
For further reading, you may be interested in the following: