Ordinary Mem is written as an open-topped shape, distinct from the fully closed square of Final Mem used at the end of a word, discussed elsewhere in this atlas; one verse preserves a famous exception to that rule. The Masoretic text of Isaiah 9:6, in a passage about an unnamed ruler's expanding dominion, spells the word lemarbeh, of the increase, with a closed final-style Mem in the middle of the word, where an open Mem is grammatically expected, an anomaly noted in the Talmud at Sanhedrin 94a.
The Talmudic passage records a tradition that the verse originally named King Hezekiah as a messianic figure, and that an angel sealed the letter shut, or the ink was miraculously fixed, to close off that reading once Hezekiah failed to fulfil it, leaving the closed Mem as a permanent trace of a redemption tradition holds was withheld.
However the anomaly arose, later scribal law simply requires it to be copied exactly as transmitted, a closed Mem standing where a plain reading of the word would call for an open one, on the strength of the received Masoretic text rather than any grammatical rule. Mem is also one of the Sefer Yetzirah's three mother letters, discussed elsewhere in this atlas, and its ordinary meaning, water, sits comfortably beside its open, flowing written form, a contrast the Isaiah anomaly makes only more noticeable by momentarily sealing a letter otherwise associated with movement and speech shut.