Its quite normal to f*ck up every now and then - its bound to happen in your private life and your academics/career at some point. Set this semester under that category and just make certain that it does not occur again. If you can do that, I think you will find that it doesn't look bad at all - it looks normal.
Also, I don't agree 'that its better sooner rather than later on account of adcoms looking favorably at an upward trend'. The only benefit of it messing up sooner is that it gives you time to show that you can handle semester-after-semester at high-performance. If you were to mess up towards the end that may suggest having burned-out.
That's all.