I witnessed a case of a talk going way over the audience's head recently at a local event. It was a technical talk, but I've seen this happen with other topics as well.
Often things like this can be avoided by checking who your audience is in advance (if in doubt, ask the organisers). In this case, the audience was foreseeably mixed and it was pretty much impossible to know in advance which way the audience would skew on that particular evening, i.e. whether there would be more technical or more non-technical people in attendance. Also, the talk was very much on-topic for the event.
