I few years ago I put Cinnamon in my coffee. After a few hours (I'm a slow coffee drinker) I started to take my last sip and a big clump of mucus hit my tongue and I almost puked. After recovering, and almost getting into fist fights with several co-workers (They loved pulling pranks) a few of them said cinnamon always does that. I'm still not so sure? What is up with Cinnamon and coffee and are there any other spices that does that? Or should I have continue to punch my co-workers?
Cinnamaldehyde, the compound that gives cinnamon its flavor, is known to cause inflammation in mucous membranes (http://cameochemicals.noaa.gov/chemical/20036).
It's also mildly water soluble. So, when you made your coffee it was probably diluted enough that it didn't cause a problem. As it sat out, it probably fell out of the solution and settled in the bottom of your cup in a more concentrated form.
Cinnamon is made from the ground bark of a variety of trees in the Cinnamomum genus. All of these barks contain starches, soluble fibers, and insoluble fibers to some degree.
Lower grade cinnamons such as Cinnamomum cassia contain higher amounts of lignins, bassorins, pectins, and mucilages; accounting for almost 80% of the mass of the powdered cinnamon. These compounds form gummy, mucus-like masses when hydrated or dissolved in water. The mucilages in cinnamon can be dissolved in cold water, but it takes heat and time to fully hydrate the bassorins and other compounds which creates a gummy mass. Higher quality cinnamon from different species may contain less mucilages or other compounds, but all powdered cinnamon will form a mass to some extent. Herbs and spices such as cardamom, chicory, nutmeg and many others contain many similar compounds in various proportions that could also form gummy masses, but it would take research and/or experimentation to figure out the amounts needed.
Cinnamon flavors can be added to coffee without ending up with sludge in your cup by adding the cinnamon to the brewing chamber or by adding cinnamon extract or cinnamon essential oil to the brewed coffee.
I know people who like cinnamon in their coffee, but the trick is to put the cinnamon in with the grounds before you brew it. I am not sure if there are other spices that do that. However, if you are looking to get back at your co-workers, just get some fine garlic powder, and sprinkle it on the base of their windshield. The air intake will suck it in, and blow it throughout their car, and the inside of their car will be smelling like garlic for a long time. The finer the powder, the better.