Looks like the Mississippi isn't the only river to have flowed backward at some point in its history:

The world's largest river, the Amazon, once flowed from the Atlantic Ocean to the Pacific - the opposite of its present direction, a study shows.

Sedimentary rocks in the central part of South America contain ancient mineral grains that must have come from the eastern part of the continent.

Geologist Russell Mapes says this must mean that about 145-65 million years ago, the Amazon flowed east to west.

The thing is, during that time period I don't think there was a land mass even vaguely resembling what we now know as South America. The fact that a river has run more or less continuously in roughly the same place through such profound changes is to me more remarkable than which way it happened to be flowing at any particular time.