The rabbis taught: It happened once that a certain child, who was reading in his teacher's house in the Book of Ezekiel, was pondering over 'Hashmal, and there came out fire from 'Hashmal and burnt him, and they sought in consequence to conceal the Book of Ezekiel. (b. Hagigah 13a)
The word "hashmal" only occurs 3 times in the Hebrew Bible, all 3 in Ezekiel's attempts to describe the appearance of God Himself. Since we don't really know what the Hebrew word was trying to describe, Jewish exegesis imbued the word itself with the dangerous power of God's presence as if the ark of the covenant itself was in the room (see 2 Sam 6:6-7 for an example). Rashi, the great medieval Jewish commentator, moves on from Ezek 1:4 with the comment that attempting to understand this verse was not allowed.
The rabbis prohibited anyone under age 30 from studying Ezekiel because of this incident. One needed to be sufficiently mature in the study of Torah before they would expound the secrets of Ezekiel 1, especially the divine chariot. So if your Bible reading plan takes you through Ezekiel, be careful and you may want to implement the buddy system. Never read alone. Just in case.