Expanding on the example given in the original post, here's an excerpt from Borel's "Essays in the History of Lie Groups and Algebraic Groups" (p. 5):

It has been remarked that, as far as terminology is concerned, posterity has not been kind to [Killing]: Cartan subalgebras, Weyl groups, fundamental reflections, roots, and the Coxeter transformation first appeared in his papers in some form. On the other hand, what now goes by his name, the "Killing form" seems to be a misnomer, and it may well be that I am the culprit. Cartan, Chevalley and Weyl never used this terminology. Once, J.J. Duistermaat and J.A.C. Kolk pointed out to me that, to their knowledge, its first occurence is in a paper of mine (Sém. Bourbaki, Exp. 33, May 1951). I do not remember why I chose it, though I probably felt I was innovating, since it is between quotation marks. It is rather likely that discussions with some members of Bourbaki had influenced me, but I cannot blame it directly on Bourbaki, since "Killing form" appears in Bourbaki drafts only from 1952 on. It is true that Killing was the first to remark that the coefficients of the characteristic equation (of a regular semisimple element), i.e. the elementary symmetric functions of the roots, are invariant under the adjoint group, but he did not make much use of the remark and did not single out the sum of the squares of the roots, of which Élie Cartan made such fundamental use in his thesis (1894). It would be more correct to speak of the Cartan form.