What are verbal errors? how do we commit such errors when we don't intend to say so?

Read the following passage and explain how wiesel interrupts the narration of the story to emphasize the truth behind what eliezer witnesses. "not far from us, flames, huge flames, were rising from a ditch. something was being burning there. a truck drew close and unloaded its hold: small children. babies! yes, 1 did see this, with my own children thrown into the frames. (is it any wonder that ever since then, sleep tends to elude me? )"