I recently found this article, which describes how...
It just so happens that the human stride has almost exactly the right frequency to drive the natural oscillations of coffee, when the fluid is in a typically sized coffee mug.
Judging by appearance, coffee and water's fluid dynamics seem rather similar. Unfortunately,they do not site a source, but they mention that the study was done by "a pair of fluid physicists at the University of California at Santa Barbara (UCSB)".
My question is how different are the properties of water and coffee? Is this difference significant enough to cause a dramatic change in how either fluid behaves in a mug (while walking)?
EDIT: Here is the actual published article.