In every interspecific relationship exhibiting commensalism, there comes a point where the organism that seems to be not affected at all, gets affected. In the relationship of the whale and barnacles, the barnacles attach to whales and they are able to transport and feed on planktons which the whale feeds on as well. This may have a negative impact on the whale due to the barnacles feeding on the same prey. Even though the effect on the whales is minimal, by definition, it would not be considered as commensalism.

Its not always about food. it can be used for transport (Phoresy) for example or Inquilinism (Permanent housing).

In every interspecific relationship exhibiting commensalism, there comes a point where the organism that seems to be not affected at all, gets affected. In the relationship of the whale and barnacles, the barnacles attach to whales and they are able to transport and feed on planktons which the whale feeds on as well. This may have a negative impact on the whale due to the barnacles feeding on the same prey. Even though the effect on the whales is minimal, by definition, it would not be considered as commensalism.

What about situations where one predator flushes prey, is unable to capture it, but that prey item is then captured by another unrelated predator?  For example, an Accipiter flushes prey from dense vegetative cover, is unable to capture it, but that prey is then captured by a falcon.  The Accipiter is not negatively affected and the falcon capitalizes on a meal it wouldn't have been able to get.  

In that case, I suppose that the Accipiter is negatively affected because it used up energy in catching its prey, which it wasn't able to catch.

I agree, but that negative affect would have been caused whether the falcon was there or not, unless the falcon does something to thwart the attack by the hawk(?).  Definitely difficult to determine if one organism in not affected in commensal relationships. 

It would be highly unlikely for a purely commensal relationship to exist in our environment. Take for example the whale barnacle and it's host whale. Most would say that it is a commensal relationship because the whale isn't outright harmed but if given more critical attention, the whale must feel an extent of discomfort, especially when numerous barnacles start to attach and proliferate on its surface; not to mention the possible added weight that it contributes for the whale to carry. Both of these may not be as significant for it to be considered as a parasitic relationship but clearly ecological relationships are not as simple as one benefits while the other is neutral in the relationship hence the simplified definition of commensalism tends to be questionable.

In commensalism, one organism benefits while the other organism is not significantly or detrimentally harmed from that relationship. There are actually two kinds of commensalism: direct and indirect. In direct commensalism, the two organisms directly interact with each other. In indirect commensalism, there is a third organism that mediates the commensalism between two organisms. An example of indirect commensalism involves a beaver (Castor canadensis), cottonwood tree (Populus spp.), and a beetle (Chrysomela confluens), wherein the cottonwood tree is the intermediary species. What happens is the beaver fells cottonwood trees, and then the beetle feeds on the leaves from the felled cottonwood trees. Specifically, the beaver makes the leaves on cottonwood trees more nutritious, enabling the beetles to grow and develop faster.