French bread falls after scoring, how can I achieve a tall loaf
If bread falls after you score it, it's because the bread is overproofed. Basically, the bubbles of carbon dioxide have reached the largest size that the structure of the dough can support and the stress of scoring causes it to collapse.  They key is to either score it earlier and let it continue rising a bit, or to score &amp; bake it earlier (this works great if you have a very hot oven and can find a way to get some steam into it). 

If your neighbor came over with bread and it flattened in your oven, it's possible that hers also overproofed in the time it took to transport it (or the extra jostling of transportation caused them to fall).