If u failed or didn't do too well in today's exam, then it'll definately have some negative effect on ur Extension 1 Maths mark. It then depends on if, after the mark is calculated, it is in your top 10 units (including English). So if it is, it'll be counted in the calculation for ur UAI and might ultimately have some unwanted effect.
However, assuming you do better in all your other subjects, and MX1 isn't counted, then your MX1 mark won't count towards anything at all in the end.
I suffered during today's exam too, so MX1 probably won't be counted for me either