I recently read how we got the name mile from a thousand Roman paces, where each pace was 5 feet. Then the English based their mile on the nearby length of 8 furlongs, which were each 10 chains, giving us the headache of 5280 feet apparently largely for tax purposes.
Here in the US, I see the word micrometer used more often to describe devices for measuring small distances than as a unit of measure; for that instead I usually see micron (μm, often rendered um).
Another fun bit of confusion seems to come with the financial suffix MM (million, explicitly "thousand thousand", used to minimize confusion internationally), which of course has nothing to do with mm. So, I try to make sure that these are correctly capitalized (or not).
Binder