When using the Calculated question type, configuring the Units to use both "ml" and "mL" produces the error "Error writing to database."
The assumption is that you can't enter two differently-cased versions of the same unit. However, the error is not handled well (my professor had no idea what had happened!). Also, if you enter only one of the two, the student is penalized by using the other.
There should be a way to enter both "ml" and "mL" as different but excepted units, as both of these are used in common scientific notation.