You might get surprise rain. The aid stations may not be stocked well. You might have hills that you didn't anticipate because the race course is "net downhill" — which doesn't mean there won't be hills; it just means that you'll finish at a lower elevation than you started. Just like in life, you can do your best to prepare, but there will still be things that are out of your control.
Image Source: POPSUGAR Photography / Emily Faulstich