Dragons
If an enemy player gets fed you can comeback by making a good play and getting shutdown gold/exp.
If an enemy team pushes hard and takes some towers you can comeback by making a good play and taking some of their towers to even things out.
If an enemy team gets dragons... you cannot come back from the power discrepancy. It's permanent. There is no comeback mechanic. Why? It isn't fun to try and finish out a map with a permanent power discrepancy.