The Arizal (1), the Gemara (2), the Rokah (3), the Sefer Hassidim (4), Rav BenTzion Abba Shaul (5), Rav Ovadia Yosef (6), Shach (7)Rav BenTzion Mutzafi Shlit”a, and the list goes on of Torah genius who warn against the improper teaching and learning of Kaballah. In my opinion is based on what I read from the Sefer “Matan Torah” by Rav Yehuda Ashlag with comments by Rav Avraham Gottleib. In the first chapter of his book, he states why he started writing all these works on Kaballah and why he made them so accessible to the common man. The problem is, people misunderstood what he was writing in that first chapter; people thought it meant that everyone should be studying Kaballah. However, he clearly stated that the purpose of his projects in Kaballah was to explain Kaballah authentically (to those who should learn), because in the past few hundred years the printing press has been very accessible to anyone who wants to write anything they can think of about just about anything.
To summarize where people went wrong: I think most people went wrong on purpose using this beginning chapter as a basis for their directionless learning of Kaballah. However, there are those who may have misinterpreted the words of this holy Sadik.
2) Hagiga 13-14b (see Maharsha to 13), Kiddushim 71 (see Maharsha), talking about learning the Shemot
3) Sefer HaShem, he was talking about the learning of Shemot
6) Yechawe Daat 4:47