|avoiding the left back corner hot spot|
Putting aside the fact that they were cupcakes, and you all know how I feel about those, the obvious thing to check would be the oven temperature.
If the batters were the same, and they were baked at the same time, but some worked and some didn't, there's probably a hot spot in her oven. That would explain why some of the cupcakes baked at different rates, and when she took them out one or two fell when others were fine.
If you put the tray of cupcakes in the oven and the ones at the back right side bake faster than the ones at the front, that might indicate where your hot spot is. If you know your oven and where the hot spots are you can rotate the pans halfway through the baking time to make sure that everything bakes evenly.
My oven has a hot spot at the back left, so if I put anything there I have to be aware that it will bake faster. Just knowing that will prevent a lot of burned cakes.
To test for hot spots in your oven you can move a thermometer around in it to see if the readings change, or use the bread trick. Take a couple of cookie sheets and cover them with slices of bread. Put the sheet in the oven and see which pieces of bread brown the fastest. That will show you where the oven is hottest, and what hot spots you need to be aware of.
Once you know where the hot spots are, you can adjust your baking plan accordingly.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA