Mom: Meggie, I let you put on your best dress this morning on one condition, that you didn't get it dirty. And look at you!
Meggie: Mum, it wasn't my fault.
Should the last sentence be "Mum, it isn't my fault."?
At the time of answering, my dress is dirty.
Wan't and isn't are pointing to the action that happened in the past or dirty dress.