The Dangers of Pitching onto a recently active yeast Cake

I brewed up a honey belgian beer a little while ago, and being the thrifty brewer I am decided to pour it directly into a recently emptied carboy with a very healthy supply of yeast from the last brew. So I lifted the kettle, put the tubing on and let it run into the carboy, after it filled I realized I had no airlock so I scampered inside to find one. This is what I found when I returned.

I filled it to the 5.5 gallon mark, and the yeast did the rest...very quickly. A bung and a blow off tube solved this issue but I learned that I should just make a starter from a previous batch and be done with it instead of trying this kind of trick again.