Rashi (Megila 25a and Berachos 34a) explains:

Can one act with Hash-m like he does with a friend -- that at times he speaks without full intent and caution and then allows himself to correct himself? With Hash-m one must use full caution from the beginning; a lapse of control when speaking to the King is not forgivable, and thus "let such a person be clubbed until he speaks with proper intent!"

All the best,

Reuven Weiner