Here's something that's been bugging me for ages.
Why do rich people and financial types write million as MM, for example, $1MM?
Someone told me once "well, the Roman numeral for thousand is M, so MM is a thousand thousand". That explanation, however, is not right. MM in Roman numerals means 2000. Million, in Roman numerals, is a single M with a bar on top. It also doesn't make sense to use the Arabic 1 plus the Roman numerals for a number.
As far as I can tell, when people write $1MM instead of $1M, it basically means "and I know lots about money and have a lot of it".