There are several papers in the literature that refer to the Pumping Lemma for context free languages as Bar-Hillel's Lemma (for example, here, here, and on the Wikipedia page). However, the first article to prove the result was only co-authored by Bar-Hillel:
Y. Bar-Hillel, M. Perles, E. Shamir. On formal properties of simple phrase structure grammars. Zeitschrift für Phonetik, Sprachwissenschaft und Kommunikationsforschung, 14:143–172, 1961.
Is there a good reason the result is sometimes called Bar-Hillel's Lemma, or is it simply a mis-attribution?