I am not sure if this is the right place to post alternate sources:

They did this very regularly in the past. And a strategy I have followed is that if I'm asked to identify the flaw and I can't see it, I look for a key term used twice. In this case selfish. If I see it clearly used in two different ways, as an adjective describing two different nouns, or with a different suffix/prefix, that was enough evidence for me to select an answer choice saying that the argument has allowed a key term to shift it's meaning over the course of the argument.

(A) is not true. That claim is relevant.
(B) is true, but premises do not need to be supported. They are simply taken for granted.
(C) is the opposite of what the argument assumes.
(D) is true, but irrelevant. The conclusion does not go beyond human behavior.
(E) is correct. The argument does allow the key term "selfish" to shift meanings over the course of the argument.

Source: LSAT, OA: E

Please explain how the word selfish is shifting in meaning? Other choices were clearly wrong.

Ok....on a second thought I think...it's not the meaning that is changing...it's the usage of the term selfish. In two instances as an adjective and in one instance as a noun -selfishness.

Now why will this be considered an error?

in general you should not expect any help on LSAT questions from MGMAT instructors on this forum. The LSAT plays by different rules than the GMAT, so studying LSAT problems is not likely to help you answer GMAT questions and may even steer you in the wrong direction..

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Tim Sanders
Manhattan GMAT Instructor