Who should you invite to speak at your conference?

A cardinal rule of organising a conference or seminar series is that you should only invite speakers who you have personally heard speak before. Otherwise, you may be unpleasantly surprised by either the content of what is presented or by the quality of delivery.
The story below concerning Karl Barth illustrates this issue. It is actually one of the most endearing stories from Bruce McCormack's seminal work, Karl Barth's Critically Realistic Dialectical Theology: Its Genesis and Development, 1909-1936.

Barth was not the first choice as speaker....those who did not know him thought of him as a follower of Leonhard Ragaz...

A more unsuitable choice for a speaker at a Religious Socialist Conference could scarcely have been found. The people gathered at Trambach had come to hear an explanation of Religious Socialisms; what they received was a sustained assault on its foundational principles.

Bruce McCormack, p. 196
You can read the full text here.