Mercury in Retrograde

For those who don't know:

Mercury in retrograde is a planetary phenomena that happens three to four times per year, when the planet Mercury slows down, and appears to stop and move backward (retrograde). It's an optical illusion, since there is forward movement, like speeding by a slow-moving train -- as it recedes, it appears to go backward.

Mercury goes into retrograde the 6th to the 29th of September.

"There's a long-held belief that it's best to avoid making set plans during the Mercury Retrograde. This means being cautious about things like signing contracts, and forming partnerships and corporations. What gets put in writing at this time may turn out to need serious revising after Mercury goes direct... "

I find this fascinating. I was just pondering what it must have looked like for a planet to go into "retrograde" to our ancestors. To those in the know it must have been seriously unsettling. Did chaos ensue? How did it become common knowledge? Who first thought it should be a time for caution?