Bernadette, DS had a two poor results in individual AS papers a couple of years back. One because he misread one of the questions and gave completely the wrong answer, and the second, a humanity, where he thought the exam was fine but then wound up with a poor result. (Not the only one, humanities seem to throw up regular surprises.)
It was difficult at the time, but looking back no harm was done. The major difference was that he applied through UCAS, with the intention of taking a gap year and reapplying if he did not get the offers he wanted, and if his grades the next summer were significantly better.
So he applied to the top four courses for his subject and a fall back. He got an offer from the fall back pretty early, a "no" from Cambridge, though did get an interview, and then was kept waiting by the other three till the end of March, which was horrid. If he had the AS grades he had expected, he probably would have got earlier offers, though Cambridge would still have been a bit of a long-shot. Instead he got an offer for the course he really wanted, possibly more than Cambridge, and indeed which is often ranked higher than Cambridge. Rejections from the other two.
Someone he knows, and in a similar position, reapplied and got Cambridge second time round (though curiously and perhaps illustrating the lottery element, was rejected by the University DS attends.)
I suspect poor results at AS is not unknown. Certainly some very bright boys, who sailed through GCSE, seemed to need a reminder that they had to shift up a gear, though I suspect this is less common with girls. Year 12 also seems to be a difficult adjustment year for dyslexics studying humanities. The silver lining is that something like this will happen at one point and that it is probably better for DC to learn about Plan Bs etc, whilst still at home. DD's school was very clear that University applications, if you are aiming for competitive courses, should be viewed as a two year process.
DD has three exams next week and then the half term break. Four more exams after, as she is taking 5 subjects. She is still very calm, and has gone out to "revise" with friends. If she pulls it off and gets the results she wants without losing sleep or losing calm, we will be very impressed. If not, and as above, she will learn some lessons.