Is there a practical limit to the amount of knowledge a human can learn? There's definitely progressive interference and retroactive interference, which are basically two ways that old memories and new memories can interfere with each other. 

But on the other hand, do these types of interference always have to happen?
Taken from a purely practical viewpoint, there is a finite number of neurons in the brain, with a finite (though large) number of connections between them. As such, a brain can only contain a finite amount of information. The more that one attempts to learn, the more that the connections between neurons will get obscured as a single memory  or idea becomes mapped to a multitude of different related pieces of information or memories.

Evolutionarily, the brain attempts to condense related information to allow for the processing of new and possibly more important data. If you tried to learn everything about every single chair in existence, your memories of different chairs would blur together as your brain tries to force similar things to be considered one memory, especially concerning things that are neither life-threatening or of obvious physical gain. This conclusion is drawn from various theories in Evolutionary Psychology, principally "The cognitive system operates at all times to optimize the adaptation of the behavior of the organism" (John R. Anderson 1990 p. 28). A short summary of these theories can be found here starting on page 783