Here is a better example than the "Group action" one, and it's a real clanger. Look at the page on "Group representation". There it defines a notion of "irreduc

Here is a better example than the "Group action" one, and it's a real clanger. Look at the page on "Group representation". There it defines a notion of "irreducible representation" in the section "Reducibility". Now, notice the phrase "A subspace W of V that is invariant under the group action is called a subrepresentation." Which group action is it talking about? This is completely orphaned. You might think to click through to the page on "Group action" and see if you can get a clue what might have been meant before an edit orphaned this phrase. In fact, a module can be thought of as a generalisation of a vector space. So in the case of an irreducible group action as defined on the "Group action" page, if R is a field, the module there is a vector space. That G acts R-linearly means precisely that there is a group homomorphism from G into GL(X), i.e. that you have a group representation into the vector space X. So, now the definition of an irreducible group action is that there is no non-trivial, proper vector subspace of X which is invariant under the group action. The definition of an irreducible group representation on the other hand states that there is no non-trivial, proper vector subspace of V that is invariant under the associated group action. But how in a million years is anyone supposed to work that out from looking at either page? And if either page changes, the connection is totally lost. A substantial part of the "Group representation" article would have to be rewritten to fix this. And similarly for the "Group action" article. Then the two would have to be connected so that a revert in one doesn't mess up all the hard work in the other. Naturally people have pointed out in 2004 that there are problems here. But in seven years it has never been fixed.