When using the <chem> tag to write a formula like "CO2 + C -> 2 CO" , the plus sign ends up as superscript to CO2, as if it was a charge (reported on it.wp). It's the first formula you see when you open the editor to add one.
And also: 2.5 should give 2 * 5, and it works. But 2 . 5, with spaces, should give 2.5, and it doesn't, so there is no way to add decimal fractions.